You could use Javascript arrays to store addtional data on the client side(browser), and then use these arrays to repolulate your downdown list by using its onchange proprty(assuming thats what u wanted to do
). This would eliminate you needing to go back to the server.
or maybe a better solution....
HTML dropdown options have an onchange proprty. you would need to use Javascript to use this event and then perform a submit on that web page. Once you have submittied to the server can then do what ever processing you need and return the html page you desire.
I have used this technique in the past when I have had a 3 or 4 option list that I wanted repopulate with differnet choices based on a values in another dropdown.
Hope this helps