camera movement problem?

when i turn the camera with

var tspeed:float=2.0;

function Start () {

}

function Update () {
if(Input.GetButtonDown("tleft")){
transform.Rotate(0, tspeed* Time.deltaTime, 0);
}
if(Input.GetButtonDown("tright")){
transform.Rotate(0, -tspeed* Time.deltaTime, 0);
}

}

it works when pressing the button once but when i hold it it doesnt turn any more and i have to press the button again.how to make it turn when i hold the key?

im using javascript.

Use Input.GetButton instead - this returns true while the button is pressed, while GetButtonDown only returns true during the frame where the button was first pressed.