diff --git a/google/auth/transport/grpc.py b/google/auth/transport/grpc.py
index 9a1bc6d..80f6e81 100644
--- a/google/auth/transport/grpc.py
+++ b/google/auth/transport/grpc.py
@@ -16,6 +16,8 @@
 
 from __future__ import absolute_import
 
+from concurrent import futures
+
 import six
 
 try:
@@ -51,6 +53,7 @@
         super(AuthMetadataPlugin, self).__init__()
         self._credentials = credentials
         self._request = request
+        self._pool = futures.ThreadPoolExecutor(max_workers=1)
 
     def _get_authorization_headers(self, context):
         """Gets the authorization headers for a request.
@@ -66,6 +69,13 @@
 
         return list(six.iteritems(headers))
 
+    @staticmethod
+    def _callback_wrapper(callback):
+        def wrapped(future):
+            callback(future.result(), None)
+
+        return wrapped
+
     def __call__(self, context, callback):
         """Passes authorization metadata into the given callback.
 
@@ -74,7 +84,11 @@
             callback (grpc.AuthMetadataPluginCallback): The callback that will
                 be invoked to pass in the authorization metadata.
         """
-        callback(self._get_authorization_headers(context), None)
+        future = self._pool.submit(self._get_authorization_headers, context)
+        future.add_done_callback(self._callback_wrapper(callback))
+
+    def __del__(self):
+        self._pool.shutdown(wait=False)
 
 
 def secure_authorized_channel(
diff --git a/google/auth/transport/requests.py b/google/auth/transport/requests.py
index 564a0cd..d1971cd 100644
--- a/google/auth/transport/requests.py
+++ b/google/auth/transport/requests.py
@@ -95,7 +95,7 @@
         self.session = session
 
     def __call__(
-        self, url, method="GET", body=None, headers=None, timeout=None, **kwargs
+        self, url, method="GET", body=None, headers=None, timeout=120, **kwargs
     ):
         """Make an HTTP request using requests.
 
diff --git a/tests/transport/test_grpc.py b/tests/transport/test_grpc.py
index 810d038..ca12385 100644
--- a/tests/transport/test_grpc.py
+++ b/tests/transport/test_grpc.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import datetime
+import time
 
 import mock
 import pytest
@@ -58,6 +59,8 @@
 
         plugin(context, callback)
 
+        time.sleep(2)
+
         callback.assert_called_once_with(
             [(u"authorization", u"Bearer {}".format(credentials.token))], None
         )
@@ -76,6 +79,8 @@
 
         plugin(context, callback)
 
+        time.sleep(2)
+
         assert credentials.token == "token1"
         callback.assert_called_once_with(
             [(u"authorization", u"Bearer {}".format(credentials.token))], None