diff --git a/gcloud/pubsub/client.py b/gcloud/pubsub/client.py
index d0c9e585539e..95164287499b 100644
--- a/gcloud/pubsub/client.py
+++ b/gcloud/pubsub/client.py
@@ -14,15 +14,33 @@
 
 """Client for interacting with the Google Cloud Pub/Sub API."""
 
+import os
 
 from gcloud.client import JSONClient
 from gcloud.pubsub.connection import Connection
-from gcloud.pubsub.connection import _PublisherAPI
-from gcloud.pubsub.connection import _SubscriberAPI
+from gcloud.pubsub.connection import _PublisherAPI as JSONPublisherAPI
+from gcloud.pubsub.connection import _SubscriberAPI as JSONSubscriberAPI
 from gcloud.pubsub.connection import _IAMPolicyAPI
 from gcloud.pubsub.subscription import Subscription
 from gcloud.pubsub.topic import Topic
 
+try:
+    from google.pubsub.v1.publisher_api import (
+        PublisherApi as GeneratedPublisherAPI)
+    from google.pubsub.v1.subscriber_api import (
+        SubscriberApi as GeneratedSubscriberAPI)
+    from gcloud.pubsub._gax import _PublisherAPI as GAXPublisherAPI
+    from gcloud.pubsub._gax import _SubscriberAPI as GAXSubscriberAPI
+except ImportError:  # pragma: NO COVER
+    _HAVE_GAX = False
+    GeneratedPublisherAPI = GAXPublisherAPI = None
+    GeneratedSubscriberAPI = GAXSubscriberAPI = None
+else:
+    _HAVE_GAX = True
+
+
+_USE_GAX = _HAVE_GAX and os.environ.get('GCLOUD_DISABLE_GAX') is None
+
 
 class Client(JSONClient):
     """Client to bundle configuration needed for API requests.
@@ -52,14 +70,22 @@ class Client(JSONClient):
     def publisher_api(self):
         """Helper for publisher-related API calls."""
         if self._publisher_api is None:
-            self._publisher_api = _PublisherAPI(self.connection)
+            if _USE_GAX:
+                generated = GeneratedPublisherAPI()
+                self._publisher_api = GAXPublisherAPI(generated)
+            else:
+                self._publisher_api = JSONPublisherAPI(self.connection)
         return self._publisher_api
 
     @property
     def subscriber_api(self):
         """Helper for subscriber-related API calls."""
         if self._subscriber_api is None:
-            self._subscriber_api = _SubscriberAPI(self.connection)
+            if _USE_GAX:
+                generated = GeneratedSubscriberAPI()
+                self._subscriber_api = GAXSubscriberAPI(generated)
+            else:
+                self._subscriber_api = JSONSubscriberAPI(self.connection)
         return self._subscriber_api
 
     @property
diff --git a/gcloud/pubsub/test_client.py b/gcloud/pubsub/test_client.py
index 7a5a54d56e87..09d2cb5841ed 100644
--- a/gcloud/pubsub/test_client.py
+++ b/gcloud/pubsub/test_client.py
@@ -29,30 +29,102 @@ def _getTargetClass(self):
     def _makeOne(self, *args, **kw):
         return self._getTargetClass()(*args, **kw)
 
-    def test_publisher_api(self):
+    def test_publisher_api_wo_gax(self):
         from gcloud.pubsub.connection import _PublisherAPI
+        from gcloud.pubsub import client as MUT
+        from gcloud._testing import _Monkey
         creds = _Credentials()
         client = self._makeOne(project=self.PROJECT, credentials=creds)
         conn = client.connection = object()
-        api = client.publisher_api
+
+        with _Monkey(MUT, _USE_GAX=False):
+            api = client.publisher_api
+
         self.assertIsInstance(api, _PublisherAPI)
         self.assertTrue(api._connection is conn)
         # API instance is cached
         again = client.publisher_api
         self.assertTrue(again is api)
 
-    def test_subscriber_api(self):
+    def test_publisher_api_w_gax(self):
+        from gcloud.pubsub import client as MUT
+        from gcloud._testing import _Monkey
+
+        wrapped = object()
+        _called_with = []
+
+        def _generated_api(*args, **kw):
+            _called_with.append((args, kw))
+            return wrapped
+
+        class _GaxPublisherAPI(object):
+
+            def __init__(self, _wrapped):
+                self._wrapped = _wrapped
+
+        creds = _Credentials()
+        client = self._makeOne(project=self.PROJECT, credentials=creds)
+
+        with _Monkey(MUT,
+                     _USE_GAX=True,
+                     GeneratedPublisherAPI=_generated_api,
+                     GAXPublisherAPI=_GaxPublisherAPI):
+            api = client.publisher_api
+
+        self.assertIsInstance(api, _GaxPublisherAPI)
+        self.assertTrue(api._wrapped is wrapped)
+        # API instance is cached
+        again = client.publisher_api
+        self.assertTrue(again is api)
+
+    def test_subscriber_api_wo_gax(self):
         from gcloud.pubsub.connection import _SubscriberAPI
+        from gcloud.pubsub import client as MUT
+        from gcloud._testing import _Monkey
         creds = _Credentials()
         client = self._makeOne(project=self.PROJECT, credentials=creds)
         conn = client.connection = object()
-        api = client.subscriber_api
+
+        with _Monkey(MUT, _USE_GAX=False):
+            api = client.subscriber_api
+
         self.assertIsInstance(api, _SubscriberAPI)
         self.assertTrue(api._connection is conn)
         # API instance is cached
         again = client.subscriber_api
         self.assertTrue(again is api)
 
+    def test_subscriber_api_w_gax(self):
+        from gcloud.pubsub import client as MUT
+        from gcloud._testing import _Monkey
+
+        wrapped = object()
+        _called_with = []
+
+        def _generated_api(*args, **kw):
+            _called_with.append((args, kw))
+            return wrapped
+
+        class _GaxSubscriberAPI(object):
+
+            def __init__(self, _wrapped):
+                self._wrapped = _wrapped
+
+        creds = _Credentials()
+        client = self._makeOne(project=self.PROJECT, credentials=creds)
+
+        with _Monkey(MUT,
+                     _USE_GAX=True,
+                     GeneratedSubscriberAPI=_generated_api,
+                     GAXSubscriberAPI=_GaxSubscriberAPI):
+            api = client.subscriber_api
+
+        self.assertIsInstance(api, _GaxSubscriberAPI)
+        self.assertTrue(api._wrapped is wrapped)
+        # API instance is cached
+        again = client.subscriber_api
+        self.assertTrue(again is api)
+
     def test_iam_policy_api(self):
         from gcloud.pubsub.connection import _IAMPolicyAPI
         creds = _Credentials()