Creating and refreshing forms with data

Hi, I'm making a database query page, using drop down menus. I'd like to make the options in each subsequent drop down menu change depending on whats selected in the first menu. I could easily use a series of pages, but Id like to keep the selections all on one page. Is there a simple way of doing this? I'm taking the first set of options from one table, and the second set from a second table (there is a relationship, so some items in the second table may or may not be related to the first table) I have already set the first menu to take it's variable from the first table. It's just the dependancy I'm stuck with. Hope this makes sense :) Regards Andrew