velocity direction

In my game, there is a ball that gets a velocity in the beginning, like this:

 var velocity : float = 15;
 function Start () {
     rigidbody.velocity = new Vector3(velocity, 0, 0,);    
 }

Now the problem is, if I rotate the ball, it keeps on going along the global x axis (Even if I rotate the ball before the game starts, it just follows global x.). How can I make the ball follow its local x axis?

Thanks

Arne