diff --git a/src/net/http/client.go b/src/net/http/client.go
index 65e0fa2bc4..6f6024ed4d 100644
--- a/src/net/http/client.go
+++ b/src/net/http/client.go
@@ -127,7 +127,10 @@ type RoundTripper interface {
 	// authentication, or cookies.
 	//
 	// RoundTrip should not modify the request, except for
-	// consuming and closing the Request's Body.
+	// consuming and closing the Request's Body. RoundTrip may
+	// read fields of the request in a separate goroutine. Callers
+	// should not mutate the request until the Response's Body has
+	// been closed.
 	//
 	// RoundTrip must always close the body, including on errors,
 	// but depending on the implementation may do so in a separate