DropDownList event issue

hi,

i want to change some input fields to uneditable when i select a value from a dropdownlist, how can i do it?

i know use AT SELECTION-SCREEN OUTPUT can change screen elements attributes,but this is triggered after you press 'enter'. when a list value selected, this event doesn't triggered.experts, would you show me what events triggered after a list valued selected? hunger for your advices, thanks!!!