Dumb question about using a <SELECT> menu to change the state of<SELECT> menu... - Javascript

This has got to be obvious, but I can't make it work.

I have a form called with 3 pull down menus. They are linked to a database
which generates the values for the <SELECT? Pull-downs.

Lets say I have values selected for all three pull down menus.

When I change the first "top-level" menu I want to reset both the second and
third menus to the "default" state.

I thought this would be easy with some javascript on the pull down in
question like:

<select name="category" onchange="document.forms.form_field.options[0];">

So when I change category I reset my sub-categories and sub-sub-categories
back to NULL to avoid duff queries being made.

I've tried any number of iterations, but it still won't work.

Please put me out of my misery?

TIA.