diff --git a/activemodel/test/cases/errors_test.rb b/activemodel/test/cases/errors_test.rb
index d10c20caad..3fa63917d0 100644
--- a/activemodel/test/cases/errors_test.rb
+++ b/activemodel/test/cases/errors_test.rb
@@ -27,6 +27,14 @@ class ErrorsTest < ActiveModel::TestCase
     end
   end
 
+  def setup
+    @mock_generator = MiniTest::Mock.new
+  end
+
+  def teardown
+    @mock_generator.verify
+  end
+
   def test_delete
     errors = ActiveModel::Errors.new(self)
     errors[:foo] << 'omg'
@@ -299,60 +307,74 @@ class ErrorsTest < ActiveModel::TestCase
 
   test "add_on_empty generates message" do
     person = Person.new
-    person.errors.expects(:generate_message).with(:name, :empty, {})
-    assert_deprecated do
-      person.errors.add_on_empty :name
+    @mock_generator.expect(:call, nil, [:name, :empty, {}])
+    person.errors.stub(:generate_message, @mock_generator) do
+      assert_deprecated do
+        person.errors.add_on_empty :name
+      end
     end
   end
 
   test "add_on_empty generates message for multiple attributes" do
     person = Person.new
-    person.errors.expects(:generate_message).with(:name, :empty, {})
-    person.errors.expects(:generate_message).with(:age, :empty, {})
-    assert_deprecated do
-      person.errors.add_on_empty [:name, :age]
+    @mock_generator.expect(:call, nil, [:name, :empty, {}])
+    @mock_generator.expect(:call, nil, [:age, :empty, {}])
+    person.errors.stub(:generate_message, @mock_generator) do
+      assert_deprecated do
+        person.errors.add_on_empty [:name, :age]
+      end
     end
   end
 
   test "add_on_empty generates message with custom default message" do
     person = Person.new
-    person.errors.expects(:generate_message).with(:name, :empty, { message: 'custom' })
-    assert_deprecated do
-      person.errors.add_on_empty :name, message: 'custom'
+    @mock_generator.expect(:call, nil, [:name, :empty, { message: 'custom' }])
+    person.errors.stub(:generate_message, @mock_generator) do
+      assert_deprecated do
+        person.errors.add_on_empty :name, message: 'custom'
+      end
     end
   end
 
   test "add_on_empty generates message with empty string value" do
     person = Person.new
     person.name = ''
-    person.errors.expects(:generate_message).with(:name, :empty, {})
-    assert_deprecated do
-      person.errors.add_on_empty :name
+    @mock_generator.expect(:call, nil, [:name, :empty, {}])
+    person.errors.stub(:generate_message, @mock_generator) do
+      assert_deprecated do
+        person.errors.add_on_empty :name
+      end
     end
   end
 
   test "add_on_blank generates message" do
     person = Person.new
-    person.errors.expects(:generate_message).with(:name, :blank, {})
-    assert_deprecated do
-      person.errors.add_on_blank :name
+    @mock_generator.expect(:call, nil, [:name, :blank, {}])
+    person.errors.stub(:generate_message, @mock_generator) do
+      assert_deprecated do
+        person.errors.add_on_blank :name
+      end
     end
   end
 
   test "add_on_blank generates message for multiple attributes" do
     person = Person.new
-    person.errors.expects(:generate_message).with(:name, :blank, {})
-    person.errors.expects(:generate_message).with(:age, :blank, {})
-    assert_deprecated do
-      person.errors.add_on_blank [:name, :age]
+    @mock_generator.expect(:call, nil, [:name, :blank, {}])
+    @mock_generator.expect(:call, nil, [:age, :blank, {}])
+    person.errors.stub(:generate_message, @mock_generator) do
+      assert_deprecated do
+        person.errors.add_on_blank [:name, :age]
+      end
     end
   end
 
   test "add_on_blank generates message with custom default message" do
     person = Person.new
-    person.errors.expects(:generate_message).with(:name, :blank, { message: 'custom' })
-    assert_deprecated do
-      person.errors.add_on_blank :name, message: 'custom'
+    @mock_generator.expect(:call, nil, [:name, :blank, { message: 'custom' }])
+    person.errors.stub(:generate_message, @mock_generator) do
+      assert_deprecated do
+        person.errors.add_on_blank :name, message: 'custom'
+      end
     end
   end
 
diff --git a/activemodel/test/cases/helper.rb b/activemodel/test/cases/helper.rb
index 2b9de5e5d2..0d179ea9ad 100644
--- a/activemodel/test/cases/helper.rb
+++ b/activemodel/test/cases/helper.rb
@@ -12,7 +12,7 @@ I18n.enforce_available_locales = false
 
 require 'active_support/testing/autorun'
 
-require 'mocha/setup' # FIXME: stop using mocha
+require 'minitest/mock'
 
 # Skips the current run on Rubinius using Minitest::Assertions#skip
 def rubinius_skip(message = '')
diff --git a/activemodel/test/cases/validations/i18n_validation_test.rb b/activemodel/test/cases/validations/i18n_validation_test.rb
index 70ee7afecc..be17575402 100644
--- a/activemodel/test/cases/validations/i18n_validation_test.rb
+++ b/activemodel/test/cases/validations/i18n_validation_test.rb
@@ -11,6 +11,7 @@ class I18nValidationTest < ActiveModel::TestCase
     I18n.load_path.clear
     I18n.backend = I18n::Backend::Simple.new
     I18n.backend.store_translations('en', errors: { messages: { custom: nil } })
+    @mock_generator = MiniTest::Mock.new
   end
 
   def teardown
@@ -18,6 +19,7 @@ class I18nValidationTest < ActiveModel::TestCase
     I18n.load_path.replace @old_load_path
     I18n.backend = @old_backend
     I18n.backend.reload!
+    @mock_generator.verify
   end
 
   def test_full_message_encoding
@@ -30,8 +32,10 @@ class I18nValidationTest < ActiveModel::TestCase
 
   def test_errors_full_messages_translates_human_attribute_name_for_model_attributes
     @person.errors.add(:name, 'not found')
-    Person.expects(:human_attribute_name).with(:name, default: 'Name').returns("Person's name")
-    assert_equal ["Person's name not found"], @person.errors.full_messages
+    @mock_generator.expect(:call, "Person's name", [:name, default: 'Name'])
+    Person.stub(:human_attribute_name, @mock_generator) do
+      assert_equal ["Person's name not found"], @person.errors.full_messages
+    end
   end
 
   def test_errors_full_messages_uses_format
@@ -60,8 +64,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_confirmation_of on generated message #{name}" do
       Person.validates_confirmation_of :title, validation_options
       @person.title_confirmation = 'foo'
-      @person.errors.expects(:generate_message).with(:title_confirmation, :confirmation, generate_message_options.merge(attribute: 'Title'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title_confirmation, :confirmation, generate_message_options.merge(attribute: 'Title')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -70,8 +76,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_acceptance_of on generated message #{name}" do
       Person.validates_acceptance_of :title, validation_options.merge(allow_nil: false)
-      @person.errors.expects(:generate_message).with(:title, :accepted, generate_message_options)
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :accepted, generate_message_options])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -80,8 +88,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_presence_of on generated message #{name}" do
       Person.validates_presence_of :title, validation_options
-      @person.errors.expects(:generate_message).with(:title, :blank, generate_message_options)
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :blank, generate_message_options])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -90,8 +100,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_length_of for :withing on generated message when too short #{name}" do
       Person.validates_length_of :title, validation_options.merge(within: 3..5)
-      @person.errors.expects(:generate_message).with(:title, :too_short, generate_message_options.merge(count: 3))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :too_short, generate_message_options.merge(count: 3)])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -101,8 +113,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_length_of for :too_long generated message #{name}" do
       Person.validates_length_of :title, validation_options.merge(within: 3..5)
       @person.title = 'this title is too long'
-      @person.errors.expects(:generate_message).with(:title, :too_long, generate_message_options.merge(count: 5))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :too_long, generate_message_options.merge(count: 5)])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -111,8 +125,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_length_of for :is on generated message #{name}" do
       Person.validates_length_of :title, validation_options.merge(is: 5)
-      @person.errors.expects(:generate_message).with(:title, :wrong_length, generate_message_options.merge(count: 5))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :wrong_length, generate_message_options.merge(count: 5)])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -122,8 +138,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_format_of on generated message #{name}" do
       Person.validates_format_of :title, validation_options.merge(with: /\A[1-9][0-9]*\z/)
       @person.title = '72x'
-      @person.errors.expects(:generate_message).with(:title, :invalid, generate_message_options.merge(value: '72x'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :invalid, generate_message_options.merge(value: '72x')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -133,8 +151,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_inclusion_of on generated message #{name}" do
       Person.validates_inclusion_of :title, validation_options.merge(in: %w(a b c))
       @person.title = 'z'
-      @person.errors.expects(:generate_message).with(:title, :inclusion, generate_message_options.merge(value: 'z'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :inclusion, generate_message_options.merge(value: 'z')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -144,8 +164,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_inclusion_of using :within on generated message #{name}" do
       Person.validates_inclusion_of :title, validation_options.merge(within: %w(a b c))
       @person.title = 'z'
-      @person.errors.expects(:generate_message).with(:title, :inclusion, generate_message_options.merge(value: 'z'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :inclusion, generate_message_options.merge(value: 'z')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -155,8 +177,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_exclusion_of generated message #{name}" do
       Person.validates_exclusion_of :title, validation_options.merge(in: %w(a b c))
       @person.title = 'a'
-      @person.errors.expects(:generate_message).with(:title, :exclusion, generate_message_options.merge(value: 'a'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :exclusion, generate_message_options.merge(value: 'a')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -166,8 +190,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_exclusion_of using :within generated message #{name}" do
       Person.validates_exclusion_of :title, validation_options.merge(within: %w(a b c))
       @person.title = 'a'
-      @person.errors.expects(:generate_message).with(:title, :exclusion, generate_message_options.merge(value: 'a'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :exclusion, generate_message_options.merge(value: 'a')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -177,8 +203,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of generated message #{name}" do
       Person.validates_numericality_of :title, validation_options
       @person.title = 'a'
-      @person.errors.expects(:generate_message).with(:title, :not_a_number, generate_message_options.merge(value: 'a'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :not_a_number, generate_message_options.merge(value: 'a')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -188,8 +216,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of for :only_integer on generated message #{name}" do
       Person.validates_numericality_of :title, validation_options.merge(only_integer: true)
       @person.title = '0.0'
-      @person.errors.expects(:generate_message).with(:title, :not_an_integer, generate_message_options.merge(value: '0.0'))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :not_an_integer, generate_message_options.merge(value: '0.0')])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -199,8 +229,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of for :odd on generated message #{name}" do
       Person.validates_numericality_of :title, validation_options.merge(only_integer: true, odd: true)
       @person.title = 0
-      @person.errors.expects(:generate_message).with(:title, :odd, generate_message_options.merge(value: 0))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :odd, generate_message_options.merge(value: 0)])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
@@ -210,8 +242,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of for :less_than on generated message #{name}" do
       Person.validates_numericality_of :title, validation_options.merge(only_integer: true, less_than: 0)
       @person.title = 1
-      @person.errors.expects(:generate_message).with(:title, :less_than, generate_message_options.merge(value: 1, count: 0))
-      @person.valid?
+      @mock_generator.expect(:call, nil, [:title, :less_than, generate_message_options.merge(value: 1, count: 0)])
+      @person.errors.stub(:generate_message, @mock_generator) do
+        @person.valid?
+      end
     end
   end
 
diff --git a/activemodel/test/cases/validations/with_validation_test.rb b/activemodel/test/cases/validations/with_validation_test.rb
index 01804032f0..9ee8b79da9 100644
--- a/activemodel/test/cases/validations/with_validation_test.rb
+++ b/activemodel/test/cases/validations/with_validation_test.rb
@@ -97,12 +97,14 @@ class ValidatesWithTest < ActiveModel::TestCase
 
   test "passes all configuration options to the validator class" do
     topic = Topic.new
-    validator = mock()
-    validator.expects(:new).with(foo: :bar, if: "1 == 1", class: Topic).returns(validator)
-    validator.expects(:validate).with(topic)
+    validator = MiniTest::Mock.new
+    validator.expect(:new, validator, [{foo: :bar, if: "1 == 1", class: Topic}])
+    validator.expect(:validate, nil, [topic])
+    validator.expect(:is_a?, false, [Symbol])
 
     Topic.validates_with(validator, if: "1 == 1", foo: :bar)
     assert topic.valid?
+    validator.verify
   end
 
   test "validates_with with options" do