diff --git a/gen/endpoints/endpoints.go b/gen/endpoints/endpoints.go
index 3fd5eb6b1f3..01ab7b2d8b7 100644
--- a/gen/endpoints/endpoints.go
+++ b/gen/endpoints/endpoints.go
@@ -10,6 +10,10 @@ import (
 // Lookup returns the endpoint for the given service in the given region plus
 // any overrides for the service name and region.
 func Lookup(service, region string) (uri, newService, newRegion string) {
+	if override := findOverride(service, region); override != nil {
+		return override.uri, override.service, override.region
+	}
+
 	switch service {
 
 	case "cloudfront":
@@ -135,9 +139,40 @@ func Lookup(service, region string) (uri, newService, newRegion string) {
 	panic("unknown endpoint for " + service + " in " + region)
 }
 
+// AddOverride overrides the endpoint for a specific service, using either an
+// existing region name or a fake one (e.g. "test-1").
+//
+// This allows developers to use local mock AWS services when they're
+// writing tests for their Go code that uses aws-go:
+//
+//     endpoints.AddOverride("EC2", "test-1", "http://localhost:3000")
+//     // This EC2 client uses the override as service endpoint.
+//     cli := ec2.New(credentials, "test-1", nil)
+func AddOverride(service, region, uri string) {
+	overrides = append(overrides, override{service, region, uri})
+}
+
 func format(uri, service, region string) string {
 	uri = strings.Replace(uri, "{scheme}", "https", -1)
 	uri = strings.Replace(uri, "{service}", service, -1)
 	uri = strings.Replace(uri, "{region}", region, -1)
 	return uri
 }
+
+func findOverride(service, region string) *override {
+	for _, override := range overrides {
+		if strings.ToUpper(override.service) == strings.ToUpper(service) &&
+			override.region == region {
+			return &override
+		}
+	}
+	return nil
+}
+
+type override struct {
+	service string
+	region  string
+	uri     string
+}
+
+var overrides []override
diff --git a/model/endpoints.go b/model/endpoints.go
index 5715633176e..ed6f1bee46b 100644
--- a/model/endpoints.go
+++ b/model/endpoints.go
@@ -134,6 +134,10 @@ import (
 // Lookup returns the endpoint for the given service in the given region plus
 // any overrides for the service name and region.
 func Lookup(service, region string) (uri, newService, newRegion string) {
+  if override := findOverride(service, region); override != nil {
+    return override.uri, override.service, override.region
+  }
+
   switch service {
     {{ range $name, $endpoints := . }}
     {{ if ne $name "_default" }}
@@ -158,10 +162,42 @@ func Lookup(service, region string) (uri, newService, newRegion string) {
   panic("unknown endpoint for " + service + " in " + region)
 }
 
+
+// AddOverride overrides the endpoint for a specific service, using either an
+// existing region name or a fake one (e.g. "test-1").
+//
+// This allows developers to use local mock AWS services when they're
+// writing tests for their Go code that uses aws-go:
+//
+//     endpoints.AddOverride("EC2", "test-1", "http://localhost:3000")
+//     // This EC2 client uses the override as service endpoint.
+//     cli := ec2.New(credentials, "test-1", nil)
+func AddOverride(service, region, uri string) {
+	overrides = append(overrides, override{service, region, uri})
+}
+
 func format(uri, service, region string) string {
   uri = strings.Replace(uri, "{scheme}", "https", -1)
   uri = strings.Replace(uri, "{service}", service, -1)
   uri = strings.Replace(uri, "{region}", region, -1)
   return uri
 }
+
+func findOverride(service, region string) *override {
+	for _, override := range overrides {
+		if strings.ToUpper(override.service) == strings.ToUpper(service) &&
+            override.region == region {
+			return &override
+		}
+	}
+	return nil
+}
+
+type override struct {
+	service string
+	region  string
+	uri     string
+}
+
+var overrides []override
 `