<script type="text/javascript" language="javascript">

    function SearchChange()

    {

        var ddl = document.getElementById('<%=DropDownList1.ClientID %>')   

        var index = ddl.selectedIndex;   

        var Value = ddl.options[index].value;   

        var Text = ddl.options[index].text;

        alert(Value);

    }

    </script>

<asp:DropDownList ID="DropDownList1" runat="server" onchange="SearchChange();">

    <asp:ListItem Value="0">111</asp:ListItem>

    <asp:ListItem Value="1">222</asp:ListItem>

    <asp:ListItem Value="2">333</asp:ListItem>

</asp:DropDownList>