SelectedIndexChanged event not working when using AJAX CascadingDropDown in ASP.Net

hi friends, can any help me?, I have two dropdownlist in cascading using a web service, all is cool, but the problem is how use the method onselectedindexchanged in the second dropdownlist, method onselectedindexchanged in second dropdownlist don't work, how could i use method selectedindexchanged in web service? some example or suggest is grateful, sorry for bad english.

thanks for read this. thanks and regards.