diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index 9d2895fb54..c877ec1708 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -747,8 +747,9 @@ class ForeignKey(ForeignObject):
 
         if on_delete is None:
             warnings.warn(
-                "on_delete will be a required arg for %s in Django 2.0. "
-                "Set it to models.CASCADE if you want to maintain the current default behavior. "
+                "on_delete will be a required arg for %s in Django 2.0. Set "
+                "it to models.CASCADE on models and in existing migrations "
+                "if you want to maintain the current default behavior. "
                 "See https://docs.djangoproject.com/en/%s/ref/models/fields/"
                 "#django.db.models.ForeignKey.on_delete" % (
                     self.__class__.__name__,
@@ -977,8 +978,9 @@ class OneToOneField(ForeignKey):
 
         if on_delete is None:
             warnings.warn(
-                "on_delete will be a required arg for %s in Django 2.0. "
-                "Set it to models.CASCADE if you want to maintain the current default behavior. "
+                "on_delete will be a required arg for %s in Django 2.0. Set "
+                "it to models.CASCADE on models and in existing migrations "
+                "if you want to maintain the current default behavior. "
                 "See https://docs.djangoproject.com/en/%s/ref/models/fields/"
                 "#django.db.models.ForeignKey.on_delete" % (
                     self.__class__.__name__,
diff --git a/tests/invalid_models_tests/test_relative_fields.py b/tests/invalid_models_tests/test_relative_fields.py
index e293087d50..f157f2beb8 100644
--- a/tests/invalid_models_tests/test_relative_fields.py
+++ b/tests/invalid_models_tests/test_relative_fields.py
@@ -53,9 +53,9 @@ class RelativeFieldTests(IsolatedModelsTestCase):
             self.assertEqual(
                 str(warns[0].message),
                 'on_delete will be a required arg for ForeignKey in Django '
-                '2.0. Set it to models.CASCADE if you want to maintain the '
-                'current default behavior. See '
-                'https://docs.djangoproject.com/en/%s/ref/models/fields/'
+                '2.0. Set it to models.CASCADE on models and in existing '
+                'migrations if you want to maintain the current default '
+                'behavior. See https://docs.djangoproject.com/en/%s/ref/models/fields/'
                 '#django.db.models.ForeignKey.on_delete' % get_docs_version(),
             )
 
@@ -90,9 +90,9 @@ class RelativeFieldTests(IsolatedModelsTestCase):
             self.assertEqual(
                 str(warns[0].message),
                 'on_delete will be a required arg for OneToOneField in Django '
-                '2.0. Set it to models.CASCADE if you want to maintain the '
-                'current default behavior. See '
-                'https://docs.djangoproject.com/en/%s/ref/models/fields/'
+                '2.0. Set it to models.CASCADE on models and in existing '
+                'migrations if you want to maintain the current default '
+                'behavior. See https://docs.djangoproject.com/en/%s/ref/models/fields/'
                 '#django.db.models.ForeignKey.on_delete' % get_docs_version(),
             )