Ajax Request with Select Box

Hi All, sorry for my silly question....
How to make an Ajax Request when the item in Select Box change ? So
when I change the item in select box I like to partial refresh some of
my page with new item choosen in select box... but I want to do it
without pressing any button at all so actually I want to use onchange
select box event, but how can I do it using prototype ?

I use observe_form for this.