diff --git a/google/appengine.go b/google/appengine.go
index b0c425d..ebdde07 100644
--- a/google/appengine.go
+++ b/google/appengine.go
@@ -8,6 +8,7 @@ import (
 	"github.com/golang/oauth2"
 
 	"appengine"
+	"appengine/urlfetch"
 )
 
 // AppEngineConfig represents a configuration for an
@@ -26,7 +27,12 @@ func NewAppEngineConfig(context appengine.Context, scopes []string) *AppEngineCo
 // NewTransport returns a transport that authorizes
 // the requests with the application's service account.
 func (c *AppEngineConfig) NewTransport() oauth2.Transport {
-	return oauth2.NewAuthorizedTransport(c, nil)
+	transport := &urlfetch.Transport{
+		Context:                       c.context,
+		Deadline:                      0,
+		AllowInvalidServerCertificate: false,
+	}
+	return oauth2.NewAuthorizedTransport(transport, c, nil)
 }
 
 // FetchToken fetches a new access token for the provided scopes.
diff --git a/google/appenginevm.go b/google/appenginevm.go
index 1885a47..96c65fe 100644
--- a/google/appenginevm.go
+++ b/google/appenginevm.go
@@ -3,6 +3,7 @@
 package google
 
 import (
+	"net/http"
 	"strings"
 
 	"github.com/golang/oauth2"
@@ -25,7 +26,7 @@ func NewAppEngineConfig(context appengine.Context, scopes []string) *AppEngineCo
 // NewTransport returns a transport that authorizes
 // the requests with the application's service account.
 func (c *AppEngineConfig) NewTransport() oauth2.Transport {
-	return oauth2.NewAuthorizedTransport(c, nil)
+	return oauth2.NewAuthorizedTransport(http.DefaultTransport, c, nil)
 }
 
 // FetchToken fetches a new access token for the provided scopes.
diff --git a/google/google.go b/google/google.go
index fec36de..292c315 100644
--- a/google/google.go
+++ b/google/google.go
@@ -61,7 +61,7 @@ func NewComputeEngineConfig(account string) *ComputeEngineConfig {
 
 // NewTransport creates an authorized transport.
 func (c *ComputeEngineConfig) NewTransport() oauth2.Transport {
-	return oauth2.NewAuthorizedTransport(c, nil)
+	return oauth2.NewAuthorizedTransport(http.DefaultTransport, c, nil)
 }
 
 // FetchToken retrieves a new access token via metadata server.
@@ -76,7 +76,7 @@ func (c *ComputeEngineConfig) FetchToken(existing *oauth2.Token) (token *oauth2.
 		return
 	}
 	req.Header.Add("X-Google-Metadata-Request", "True")
-	resp, err := (&http.Client{Transport: oauth2.DefaultTransport}).Do(req)
+	resp, err := (&http.Client{}).Do(req)
 	if err != nil {
 		return
 	}
diff --git a/jwt.go b/jwt.go
index 5bda21b..d35be52 100644
--- a/jwt.go
+++ b/jwt.go
@@ -61,13 +61,13 @@ type JWTConfig struct {
 // NewTransport creates a transport that is authorize with the
 // parent JWT configuration.
 func (c *JWTConfig) NewTransport() Transport {
-	return NewAuthorizedTransport(c, &Token{})
+	return NewAuthorizedTransport(http.DefaultTransport, c, &Token{})
 }
 
 // NewTransportWithUser creates a transport that is authorized by
 // the client and impersonates the specified user.
 func (c *JWTConfig) NewTransportWithUser(user string) Transport {
-	return NewAuthorizedTransport(c, &Token{Subject: user})
+	return NewAuthorizedTransport(http.DefaultTransport, c, &Token{Subject: user})
 }
 
 // fetchToken retrieves a new access token and updates the existing token
@@ -100,7 +100,7 @@ func (c *JWTConfig) FetchToken(existing *Token) (token *Token, err error) {
 	v.Set("assertion", payload)
 
 	//  Make a request with assertion to get a new token.
-	client := http.Client{Transport: DefaultTransport}
+	client := http.Client{}
 	resp, err := client.PostForm(c.aud, v)
 	if err != nil {
 		return nil, err
diff --git a/oauth2.go b/oauth2.go
index 4642627..2d3d778 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -18,10 +18,6 @@ import (
 	"time"
 )
 
-// The default transport implementation to be used while
-// making the authorized requests.
-var DefaultTransport = http.DefaultTransport
-
 type tokenRespBody struct {
 	AccessToken  string        `json:"access_token"`
 	TokenType    string        `json:"token_type"`
@@ -138,7 +134,7 @@ func (c *Config) AuthCodeURL(state string) (authURL string, err error) {
 //     t.SetToken(validToken)
 //
 func (c *Config) NewTransport() Transport {
-	return NewAuthorizedTransport(c, nil)
+	return NewAuthorizedTransport(http.DefaultTransport, c, nil)
 }
 
 // NewTransportWithCode exchanges the OAuth 2.0 exchange code with
@@ -150,7 +146,7 @@ func (c *Config) NewTransportWithCode(exchangeCode string) (Transport, error) {
 	if err != nil {
 		return nil, err
 	}
-	return NewAuthorizedTransport(c, token), nil
+	return NewAuthorizedTransport(http.DefaultTransport, c, token), nil
 }
 
 // FetchToken retrieves a new access token and updates the existing token
@@ -210,7 +206,7 @@ func (c *Config) exchange(exchangeCode string) (*Token, error) {
 func (c *Config) updateToken(tok *Token, v url.Values) error {
 	v.Set("client_id", c.opts.ClientID)
 	v.Set("client_secret", c.opts.ClientSecret)
-	r, err := (&http.Client{Transport: DefaultTransport}).PostForm(c.tokenURL, v)
+	r, err := (&http.Client{}).PostForm(c.tokenURL, v)
 	if err != nil {
 		return err
 	}
diff --git a/oauth2_test.go b/oauth2_test.go
index e734a83..b09a813 100644
--- a/oauth2_test.go
+++ b/oauth2_test.go
@@ -35,8 +35,6 @@ func newTestConf() *Config {
 }
 
 func TestAuthCodeURL(t *testing.T) {
-	DefaultTransport = http.DefaultTransport
-
 	conf := newTestConf()
 	url, err := conf.AuthCodeURL("foo")
 	if err != nil {
@@ -48,8 +46,13 @@ func TestAuthCodeURL(t *testing.T) {
 }
 
 func TestExchangePayload(t *testing.T) {
+	oldDefaultTransport := http.DefaultTransport
+	defer func() {
+		http.DefaultTransport = oldDefaultTransport
+	}()
+
 	conf := newTestConf()
-	DefaultTransport = &mockTransport{
+	http.DefaultTransport = &mockTransport{
 		rt: func(req *http.Request) (resp *http.Response, err error) {
 			headerContentType := req.Header.Get("Content-Type")
 			if headerContentType != "application/x-www-form-urlencoded" {
@@ -66,8 +69,13 @@ func TestExchangePayload(t *testing.T) {
 }
 
 func TestExchangingTransport(t *testing.T) {
+	oldDefaultTransport := http.DefaultTransport
+	defer func() {
+		http.DefaultTransport = oldDefaultTransport
+	}()
+
 	conf := newTestConf()
-	DefaultTransport = &mockTransport{
+	http.DefaultTransport = &mockTransport{
 		rt: func(req *http.Request) (resp *http.Response, err error) {
 			if req.URL.RequestURI() != "token-url" {
 				t.Fatalf("NewTransportWithCode should have exchanged the code, but it didn't.")
@@ -79,8 +87,6 @@ func TestExchangingTransport(t *testing.T) {
 }
 
 func TestFetchWithNoRedirect(t *testing.T) {
-	DefaultTransport = http.DefaultTransport
-
 	fetcher := newTestConf()
 	_, err := fetcher.FetchToken(&Token{})
 	if err == nil {
diff --git a/transport.go b/transport.go
index 39726b0..cf72dab 100644
--- a/transport.go
+++ b/transport.go
@@ -76,8 +76,9 @@ type Transport interface {
 }
 
 type authorizedTransport struct {
-	fetcher TokenFetcher
-	token   *Token
+	fetcher       TokenFetcher
+	token         *Token
+	origTransport http.RoundTripper
 
 	// Mutex to protect token during auto refreshments.
 	mu sync.RWMutex
@@ -86,8 +87,8 @@ type authorizedTransport struct {
 // NewAuthorizedTransport creates a transport that uses the provided
 // token fetcher to retrieve new tokens if there is no access token
 // provided or it is expired.
-func NewAuthorizedTransport(fetcher TokenFetcher, token *Token) Transport {
-	return &authorizedTransport{fetcher: fetcher, token: token}
+func NewAuthorizedTransport(origTransport http.RoundTripper, fetcher TokenFetcher, token *Token) Transport {
+	return &authorizedTransport{origTransport: origTransport, fetcher: fetcher, token: token}
 }
 
 // RoundTrip authorizes the request with the existing token.
@@ -118,7 +119,7 @@ func (t *authorizedTransport) RoundTrip(req *http.Request) (resp *http.Response,
 	req.Header.Set("Authorization", typ+" "+token.AccessToken)
 
 	// Make the HTTP request.
-	return DefaultTransport.RoundTrip(req)
+	return t.origTransport.RoundTrip(req)
 }
 
 // Token returns the existing token that authorizes the Transport.
diff --git a/transport_test.go b/transport_test.go
index 847266a..e5bfe6f 100644
--- a/transport_test.go
+++ b/transport_test.go
@@ -14,7 +14,7 @@ func (f *mockTokenFetcher) FetchToken(existing *Token) (*Token, error) {
 }
 
 func TestInitialTokenRead(t *testing.T) {
-	tr := NewAuthorizedTransport(nil, &Token{AccessToken: "abc"})
+	tr := NewAuthorizedTransport(http.DefaultTransport, nil, &Token{AccessToken: "abc"})
 	server := newMockServer(func(w http.ResponseWriter, r *http.Request) {
 		if r.Header.Get("Authorization") != "Bearer abc" {
 			t.Errorf("Transport doesn't set the Authorization header from the initial token")
@@ -31,7 +31,7 @@ func TestTokenFetch(t *testing.T) {
 			AccessToken: "abc",
 		},
 	}
-	tr := NewAuthorizedTransport(fetcher, nil)
+	tr := NewAuthorizedTransport(http.DefaultTransport, fetcher, nil)
 	server := newMockServer(func(w http.ResponseWriter, r *http.Request) {
 		if r.Header.Get("Authorization") != "Bearer abc" {
 			t.Errorf("Transport doesn't set the Authorization header from the fetched token")