Hide the Popup control extender when i select the value in dropdown

To hide it with javascript, first assign a behaviorID to the extender. Then use the following code to close it.

your popup extender tag:

<ajaxToolkit:PopupControlExtender ID="PopupControlExtender1" BehaviorID="pop" runat="server" PopupControlID="Panel1" TargetControlID="Button1">
     </ajaxToolkit:PopupControlExtender>

call hide() functipon in onChange event of your drop down.

function hide()
{
     var e = document.getElementById("dropdown_Id");
     var strUser = e.options[e.selectedIndex].value;
     //i ssumed first element should be blank
     if(strUser!='')
     {       
        var bh = $find("pop");
        bh.hidePopup();
     }
     else
     {
        return false;
     }
}