diff --git a/src/pkg/sync/rwmutex_test.go b/src/pkg/sync/rwmutex_test.go
index 9fb89f8e8a..0480a66018 100644
--- a/src/pkg/sync/rwmutex_test.go
+++ b/src/pkg/sync/rwmutex_test.go
@@ -45,6 +45,7 @@ func doTestParallelReaders(numReaders, gomaxprocs int) {
 }
 
 func TestParallelReaders(t *testing.T) {
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(-1))
 	doTestParallelReaders(1, 4)
 	doTestParallelReaders(3, 4)
 	doTestParallelReaders(4, 2)
@@ -102,6 +103,7 @@ func HammerRWMutex(gomaxprocs, numReaders, num_iterations int) {
 }
 
 func TestRWMutex(t *testing.T) {
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(-1))
 	n := 1000
 	if testing.Short() {
 		n = 5
diff --git a/src/pkg/testing/benchmark.go b/src/pkg/testing/benchmark.go
index db4c65941c..f8b53e63ae 100644
--- a/src/pkg/testing/benchmark.go
+++ b/src/pkg/testing/benchmark.go
@@ -143,7 +143,6 @@ func (b *B) run() BenchmarkResult {
 		b.runN(n)
 	}
 	return BenchmarkResult{b.N, b.ns, b.bytes}
-
 }
 
 // The results of a benchmark run.
@@ -183,6 +182,7 @@ func RunBenchmarks(matchString func(pat, str string) (bool, os.Error), benchmark
 	if len(*matchBenchmarks) == 0 {
 		return
 	}
+	procs := runtime.GOMAXPROCS(-1)
 	for _, Benchmark := range benchmarks {
 		matched, err := matchString(*matchBenchmarks, Benchmark.Name)
 		if err != nil {
@@ -194,7 +194,12 @@ func RunBenchmarks(matchString func(pat, str string) (bool, os.Error), benchmark
 		}
 		b := &B{benchmark: Benchmark}
 		r := b.run()
-		fmt.Printf("%s\t%v\n", Benchmark.Name, r)
+		print(fmt.Sprintf("%s\t%v\n", Benchmark.Name, r))
+		if p := runtime.GOMAXPROCS(-1); p != procs {
+			print(fmt.Sprintf("%s left GOMAXPROCS set to %d\n", Benchmark.Name, p))
+			procs = p
+		}
+
 	}
 }
 
diff --git a/src/pkg/testing/testing.go b/src/pkg/testing/testing.go
index 8781b207de..3b2dd377af 100644
--- a/src/pkg/testing/testing.go
+++ b/src/pkg/testing/testing.go
@@ -171,6 +171,7 @@ func RunTests(matchString func(pat, str string) (bool, os.Error), tests []Intern
 	if len(tests) == 0 {
 		println("testing: warning: no tests to run")
 	}
+	procs := runtime.GOMAXPROCS(-1)
 	for i := 0; i < len(tests); i++ {
 		matched, err := matchString(*match, tests[i].Name)
 		if err != nil {
@@ -190,6 +191,11 @@ func RunTests(matchString func(pat, str string) (bool, os.Error), tests []Intern
 		<-t.ch
 		ns += time.Nanoseconds()
 		tstr := fmt.Sprintf("(%.2f seconds)", float64(ns)/1e9)
+		if p := runtime.GOMAXPROCS(-1); t.failed == false && p != procs {
+			t.failed = true
+			t.errors = fmt.Sprintf("%s left GOMAXPROCS set to %d\n", tests[i].Name, p)
+			procs = p
+		}
 		if t.failed {
 			println("--- FAIL:", tests[i].Name, tstr)
 			print(t.errors)