Using Ajax to populate a drop-down list

Hi, I have a form with two lists. One is populated with many options while the second is populated with only a single item. When the first drop-down list is changed, I call a PHP script sing AJAX to populate the second list. I also need to perform another task when the second list is changed but it seems not to work. I think this is because even though my AJAX script populates the second drop-down list using pure PHP code, the second drop-down isn't aware of it. I've tries adding JavaScript code in the AJAX route to add options to the list but it's not working. Does anyone have an example of how to do this? Thanks, Don