diff --git a/builtin_math.go b/builtin_math.go
index a9f4a55..7ce90c3 100644
--- a/builtin_math.go
+++ b/builtin_math.go
@@ -117,7 +117,13 @@ func builtinMath_pow(call FunctionCall) Value {
 }
 
 func builtinMath_random(call FunctionCall) Value {
-	return toValue_float64(rand.Float64())
+	var v float64
+	if call.runtime.random != nil {
+		v = call.runtime.random()
+	} else {
+		v = rand.Float64()
+	}
+	return toValue_float64(v)
 }
 
 func builtinMath_round(call FunctionCall) Value {
diff --git a/otto.go b/otto.go
index 2ec033c..6135330 100644
--- a/otto.go
+++ b/otto.go
@@ -363,6 +363,10 @@ func (self Otto) SetDebuggerHandler(fn func(vm *Otto)) {
 	self.runtime.debugger = fn
 }
 
+func (self Otto) SetRandomSource(fn func() float64) {
+	self.runtime.random = fn
+}
+
 // Context is a structure that contains information about the current execution
 // context.
 type Context struct {
diff --git a/runtime.go b/runtime.go
index 168cb1c..a998f7a 100644
--- a/runtime.go
+++ b/runtime.go
@@ -55,6 +55,7 @@ type _runtime struct {
 	otto         *Otto
 	eval         *_object // The builtin eval, for determine indirect versus direct invocation
 	debugger     func(*Otto)
+	random       func() float64
 
 	labels []string // FIXME
 	lck    sync.Mutex
diff --git a/runtime_test.go b/runtime_test.go
index 247ecab..eeb1173 100644
--- a/runtime_test.go
+++ b/runtime_test.go
@@ -803,3 +803,32 @@ func Test_debugger(t *testing.T) {
 		is(err, nil)
 	})
 }
+
+func Test_random(t *testing.T) {
+	tt(t, func() {
+		vm := New()
+		vm.SetRandomSource(func() float64 { return 1 })
+
+		r, err := vm.Run(`Math.random()`)
+		is(err, nil)
+		f, err := r.ToFloat()
+		is(err, nil)
+		is(f, 1)
+	})
+
+	tt(t, func() {
+		vm := New()
+
+		r1, err := vm.Run(`Math.random()`)
+		is(err, nil)
+		f1, err := r1.ToFloat()
+		is(err, nil)
+
+		r2, err := vm.Run(`Math.random()`)
+		is(err, nil)
+		f2, err := r2.ToFloat()
+		is(err, nil)
+
+		is(f1 == f2, false)
+	})
+}