diff -r 50a5e271edf9 Lib/json/tests/test_recursion.py
--- a/Lib/json/tests/test_recursion.py	Tue May 10 00:39:02 2011 -0700
+++ b/Lib/json/tests/test_recursion.py	Tue May 10 22:08:35 2011 +0300
@@ -16,6 +16,11 @@
                 return 'JSONTestObject'
         return json.JSONEncoder.default(o)
 
+class EndlessJSONEncoder(json.JSONEncoder):
+    def default(self, o):
+        """If check_circular is False, this will keep adding another list."""
+        return [o]
+
 
 class TestRecursion(TestCase):
     def test_listrecursion(self):
@@ -67,7 +72,7 @@
             self.fail("didn't raise ValueError on default recursion")
 
 
-    def test_highly_nested_objects(self):
+    def test_highly_nested_objects_decoding(self):
         # test that loading highly-nested objects doesn't segfault when C
         # accelerations are used. See #12017
         # str
@@ -84,3 +89,18 @@
             json.loads(u'{"a":' * 100000 + u'[1]' + u'}' * 100000)
         with self.assertRaises(RuntimeError):
             json.loads(u'[' * 100000 + u'1' + u']' * 100000)
+
+    def test_highly_nested_objects_encoding(self):
+        # See #12051
+        l, d = [], {}
+        for x in xrange(100000):
+            l, d = [l], {'k':d}
+        with self.assertRaises(RuntimeError):
+            json.dumps(l)
+        with self.assertRaises(RuntimeError):
+            json.dumps(d)
+
+    def test_endless_recursion(self):
+        # See #12051
+        with self.assertRaises(RuntimeError):
+            EndlessJSONEncoder(check_circular=False).encode(5j)
diff -r 50a5e271edf9 Modules/_json.c
--- a/Modules/_json.c	Tue May 10 00:39:02 2011 -0700
+++ b/Modules/_json.c	Tue May 10 22:08:35 2011 +0300
@@ -1999,10 +1999,18 @@
         return _steal_list_append(rval, encoded);
     }
     else if (PyList_Check(obj) || PyTuple_Check(obj)) {
-        return encoder_listencode_list(s, rval, obj, indent_level);
+        if (Py_EnterRecursiveCall(" while encoding a JSON object"))
+            return -1;
+        rv = encoder_listencode_list(s, rval, obj, indent_level);
+        Py_LeaveRecursiveCall();
+        return rv;
     }
     else if (PyDict_Check(obj)) {
-        return encoder_listencode_dict(s, rval, obj, indent_level);
+        if (Py_EnterRecursiveCall(" while encoding a JSON object"))
+            return -1;
+        rv = encoder_listencode_dict(s, rval, obj, indent_level);
+        Py_LeaveRecursiveCall();
+        return rv;
     }
     else {
         PyObject *ident = NULL;
@@ -2028,7 +2036,12 @@
             Py_XDECREF(ident);
             return -1;
         }
+
+        if (Py_EnterRecursiveCall(" while encoding a JSON object"))
+            return -1;
         rv = encoder_listencode_obj(s, rval, newobj, indent_level);
+        Py_LeaveRecursiveCall();
+
         Py_DECREF(newobj);
         if (rv) {
             Py_XDECREF(ident);