Inital Animation

Hi.
I'd like to know how I can produce an initial animation.
I want animate a div automatically when the page is opened.

I can animate my div associating the animation, for example, to a button or to a mouseover action, but i can't create an automatic animation.

I use a simple code

$(window).load(function(){
$("#mydiv").animate({width:"800"},slow)
});

Can you help me please?