VS 2008 EXecuting JS On Form Load

I want the javascript function to be executed on the from load. SUppose I want the JS to be executed on the button click,say for example simple Pop Up. For dat we use the code

Code:
protected void Page_Load(object sender, EventArgs e)
{
Button1.Attributes.Add("onclick", "javascript : alert1();");
}

Code:

<script language ="javascript" type ="text/javascript" >
function alert1()
{
alert('Popup');
}
</script>

Can somebody tell me how to execute the JS on the form load?