diff --git a/google/auth/compute_engine/_metadata.py b/google/auth/compute_engine/_metadata.py
index e5abf20..b35775a 100644
--- a/google/auth/compute_engine/_metadata.py
+++ b/google/auth/compute_engine/_metadata.py
@@ -26,15 +26,18 @@
 from six.moves.urllib import parse as urlparse
 
 from google.auth import _helpers
+from google.auth import environment_vars
 from google.auth import exceptions
 
 _LOGGER = logging.getLogger(__name__)
 
-_METADATA_ROOT = 'http://metadata.google.internal/computeMetadata/v1/'
+_METADATA_ROOT = 'http://{}/computeMetadata/v1/'.format(
+    os.getenv(environment_vars.GCE_METADATA_ROOT, 'metadata.google.internal'))
 
 # This is used to ping the metadata server, it avoids the cost of a DNS
 # lookup.
-_METADATA_IP_ROOT = 'http://169.254.169.254'
+_METADATA_IP_ROOT = 'http://{}'.format(
+    os.getenv(environment_vars.GCE_METADATA_IP, '169.254.169.254'))
 _METADATA_FLAVOR_HEADER = 'metadata-flavor'
 _METADATA_FLAVOR_VALUE = 'Google'
 _METADATA_HEADERS = {_METADATA_FLAVOR_HEADER: _METADATA_FLAVOR_VALUE}
diff --git a/google/auth/environment_vars.py b/google/auth/environment_vars.py
index b4ed2b2..0110e6a 100644
--- a/google/auth/environment_vars.py
+++ b/google/auth/environment_vars.py
@@ -37,3 +37,13 @@
 CLOUD_SDK_CONFIG_DIR = 'CLOUDSDK_CONFIG'
 """Environment variable defines the location of Google Cloud SDK's config
 files."""
+
+# These two variables allow for customization of the addresses used when
+# contacting the GCE metadata service.
+GCE_METADATA_ROOT = 'GCE_METADATA_ROOT'
+"""Environment variable providing an alternate hostname or host:port to be
+used for GCE metadata requests."""
+
+GCE_METADATA_IP = 'GCE_METADATA_IP'
+"""Environment variable providing an alternate ip:port to be used for ip-only
+GCE metadata requests."""