diff --git a/requests/auth.py b/requests/auth.py
index f87087df..6664cd80 100644
--- a/requests/auth.py
+++ b/requests/auth.py
@@ -16,6 +16,7 @@ import logging
 from base64 import b64encode
 
 from .compat import urlparse, str
+from .cookies import extract_cookies_to_jar
 from .utils import parse_dict_header
 
 log = logging.getLogger(__name__)
@@ -169,7 +170,8 @@ class HTTPDigestAuth(AuthBase):
             r.content
             r.raw.release_conn()
             prep = r.request.copy()
-            prep.prepare_cookies(r.cookies)
+            extract_cookies_to_jar(prep._cookies, r.request, r.raw)
+            prep.prepare_cookies(prep._cookies)
 
             prep.headers['Authorization'] = self.build_digest_header(
                 prep.method, prep.url)
diff --git a/requests/models.py b/requests/models.py
index c68f9118..34dce181 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -270,6 +270,9 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         self.url = None
         #: dictionary of HTTP headers.
         self.headers = None
+        # The `CookieJar` used to create the Cookie header will be stored here
+        # after prepare_cookies is called
+        self._cookies = None
         #: request body to send to the server.
         self.body = None
         #: dictionary of callback hooks, for internal usage.
@@ -299,6 +302,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         p.method = self.method
         p.url = self.url
         p.headers = self.headers.copy()
+        p._cookies = self._cookies.copy()
         p.body = self.body
         p.hooks = self.hooks
         return p
@@ -474,14 +478,13 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         """Prepares the given HTTP cookie data."""
 
         if isinstance(cookies, cookielib.CookieJar):
-            cookies = cookies
+            self._cookies = cookies
         else:
-            cookies = cookiejar_from_dict(cookies)
+            self._cookies = cookiejar_from_dict(cookies)
 
-        if 'cookie' not in self.headers:
-            cookie_header = get_cookie_header(cookies, self)
-            if cookie_header is not None:
-                self.headers['Cookie'] = cookie_header
+        cookie_header = get_cookie_header(self._cookies, self)
+        if cookie_header is not None:
+            self.headers['Cookie'] = cookie_header
 
     def prepare_hooks(self, hooks):
         """Prepares the given hooks."""
diff --git a/requests/sessions.py b/requests/sessions.py
index cdce6484..06e17d4b 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -153,7 +153,9 @@ class SessionRedirectMixin(object):
             except KeyError:
                 pass
 
-            prepared_request.prepare_cookies(self.cookies)
+            extract_cookies_to_jar(prepared_request._cookies,
+                                   prepared_request, resp.raw)
+            prepared_request.prepare_cookies(prepared_request._cookies)
 
             resp = self.send(
                 prepared_request,
@@ -345,9 +347,6 @@ class Session(SessionRedirectMixin):
         )
         prep = self.prepare_request(req)
 
-        # Add param cookies to session cookies
-        self.cookies = merge_cookies(self.cookies, cookies)
-
         proxies = proxies or {}
 
         # Gather clues from the surrounding environment.
diff --git a/test_requests.py b/test_requests.py
index 1d68c81f..617e6a7f 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -165,7 +165,7 @@ class RequestsTestCase(unittest.TestCase):
 
     def test_cookie_persists_via_api(self):
         s = requests.session()
-        r = s.get(httpbin('redirect/1'), cookies={'foo':'bar'})
+        r = s.get(httpbin('redirect/1'), cookies={'foo': 'bar'})
         assert 'foo' in r.request.headers['Cookie']
         assert 'foo' in r.history[0].request.headers['Cookie']
 
@@ -177,6 +177,12 @@ class RequestsTestCase(unittest.TestCase):
         # Session cookie should not be modified
         assert s.cookies['foo'] == 'bar'
 
+    def test_request_cookies_not_persisted(self):
+        s = requests.session()
+        s.get(httpbin('cookies'), cookies={'foo': 'baz'})
+        # Sending a request with cookies should not add cookies to the session
+        assert not s.cookies
+
     def test_generic_cookiejar_works(self):
         cj = cookielib.CookieJar()
         cookiejar_from_dict({'foo': 'bar'}, cj)