<asp:DropDownList ID="DropdownList1" runat="server" AutoPostBack="true" />

 <script type="text/JavaScript">

 function getvalue(){

                 var select1 = document.all.<%= DropdownList1.ClientID %>;

                 var selectvalue = select1.options[select1.selectedIndex].value;

                 var selectText= select1.options[select1.selectedIndex].text;

                alert(selectvalue+"  "+selectText);

 }

 </script>