diff --git a/activemodel/lib/active_model/validations/inclusion.rb b/activemodel/lib/active_model/validations/inclusion.rb
index 049b093618..108586b8df 100644
--- a/activemodel/lib/active_model/validations/inclusion.rb
+++ b/activemodel/lib/active_model/validations/inclusion.rb
@@ -8,9 +8,26 @@ module ActiveModel
                               ":in option of the configuration hash" unless options[:in].respond_to?(:include?)
       end
 
-      def validate_each(record, attribute, value)
-        unless options[:in].include?(value)
-          record.errors.add(attribute, :inclusion, options.except(:in).merge!(:value => value))
+      # On Ruby 1.9 Range#include? checks all possible values in the range for equality,
+      # so it may be slow for large ranges. The new Range#cover? uses the previous logic
+      # of comparing a value with the range endpoints.
+      if (1..2).respond_to?(:cover?)
+        def validate_each(record, attribute, value)
+          included = if options[:in].is_a?(Range)
+            options[:in].cover?(value)
+          else
+            options[:in].include?(value)
+          end
+
+          unless included
+            record.errors.add(attribute, :inclusion, options.except(:in).merge!(:value => value))
+          end
+        end
+      else
+        def validate_each(record, attribute, value)
+          unless options[:in].include?(value)
+            record.errors.add(attribute, :inclusion, options.except(:in).merge!(:value => value))
+          end
         end
       end
     end
diff --git a/activemodel/test/cases/validations/inclusion_validation_test.rb b/activemodel/test/cases/validations/inclusion_validation_test.rb
index 0716b4f087..62f2ec785d 100644
--- a/activemodel/test/cases/validations/inclusion_validation_test.rb
+++ b/activemodel/test/cases/validations/inclusion_validation_test.rb
@@ -10,6 +10,15 @@ class InclusionValidationTest < ActiveModel::TestCase
     Topic.reset_callbacks(:validate)
   end
 
+  def test_validates_inclusion_of_range
+    Topic.validates_inclusion_of( :title, :in => 'aaa'..'bbb' )
+    assert Topic.new("title" => "bbc", "content" => "abc").invalid?
+    assert Topic.new("title" => "aa", "content" => "abc").invalid?
+    assert Topic.new("title" => "aaa", "content" => "abc").valid?
+    assert Topic.new("title" => "abc", "content" => "abc").valid?
+    assert Topic.new("title" => "bbb", "content" => "abc").valid?
+  end
+
   def test_validates_inclusion_of
     Topic.validates_inclusion_of( :title, :in => %w( a b c d e f g ) )