diff --git a/requests/models.py b/requests/models.py
index 3f6a4f92..2370b67f 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -472,7 +472,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
             l = super_len(body)
             if l:
                 self.headers['Content-Length'] = builtin_str(l)
-        elif self.method not in ('GET', 'HEAD'):
+        elif (self.method not in ('GET', 'HEAD')) and (self.headers.get('Content-Length') is None):
             self.headers['Content-Length'] = '0'
 
     def prepare_auth(self, auth, url=''):
diff --git a/test_requests.py b/test_requests.py
index b2e12d06..4a05cb2e 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -103,6 +103,14 @@ class RequestsTestCase(unittest.TestCase):
         head_req = requests.Request('HEAD', httpbin('head')).prepare()
         assert 'Content-Length' not in head_req.headers
 
+    def test_override_content_length(self):
+        headers = {
+            'Content-Length': 'not zero'
+        }
+        r = requests.Request('POST', httpbin('post'), headers=headers).prepare()
+        assert 'Content-Length' in r.headers
+        assert r.headers['Content-Length'] == 'not zero'
+
     def test_path_is_not_double_encoded(self):
         request = requests.Request('GET', "http://0.0.0.0/get/test case").prepare()