diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index ccf5605a63..bb28756133 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1799,9 +1799,9 @@
 //	    the package list (if present) must appear before this flag.
 //
 //	-c
-//	    Compile the test binary to pkg.test but do not run it
+//	    Compile the test binary to pkg.test in the current directory but do not run it
 //	    (where pkg is the last element of the package's import path).
-//	    The file name can be changed with the -o flag.
+//	    The file name or target directory can be changed with the -o flag.
 //
 //	-exec xprog
 //	    Run the test binary using xprog. The behavior is the same as
@@ -1814,6 +1814,8 @@
 //	-o file
 //	    Compile the test binary to the named file.
 //	    The test still runs (unless -c or -i is specified).
+//	    If file ends in a slash or names an existing directory,
+//	    the test is written to pkg.test in that directory.
 //
 // The test binary also accepts flags that control execution of the test; these
 // flags are also accessible by 'go test'. See 'go help testflag' for details.
diff --git a/src/cmd/go/internal/test/test.go b/src/cmd/go/internal/test/test.go
index cb8cb83701..2ce4c1a28e 100644
--- a/src/cmd/go/internal/test/test.go
+++ b/src/cmd/go/internal/test/test.go
@@ -145,9 +145,9 @@ In addition to the build flags, the flags handled by 'go test' itself are:
 	    the package list (if present) must appear before this flag.
 
 	-c
-	    Compile the test binary to pkg.test but do not run it
+	    Compile the test binary to pkg.test in the current directory but do not run it
 	    (where pkg is the last element of the package's import path).
-	    The file name can be changed with the -o flag.
+	    The file name or target directory can be changed with the -o flag.
 
 	-exec xprog
 	    Run the test binary using xprog. The behavior is the same as
@@ -160,6 +160,8 @@ In addition to the build flags, the flags handled by 'go test' itself are:
 	-o file
 	    Compile the test binary to the named file.
 	    The test still runs (unless -c or -i is specified).
+	    If file ends in a slash or names an existing directory,
+	    the test is written to pkg.test in that directory.
 
 The test binary also accepts flags that control execution of the test; these
 flags are also accessible by 'go test'. See 'go help testflag' for details.