<select id="mySelect" name="mySelect" runat="server" AutoPostBack="true"  OnSeletedIndexChanged="mySelect_SelectIndexChanged"></select>

<asp:DropDownList id="AutosList" AutoPostBack="True" OnSelectedIndexChanged="AutosList_SelectIndexChanged"
                    runat="server">
      <asp:ListItem Value="1">Ford</asp:ListItem>
      <asp:ListItem Value="2">Toyota</asp:ListItem>
      <asp:ListItem Value="3">Ferrari</asp:ListItem>
 </asp:DropDownList>