<form id="form1" runat="server">
        <asp:ScriptManager ID="ScriptManager1" runat="server" />
        <asp:RadioButtonList ID="RadioButtonList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="RadioButtonList1_SelectedIndexChanged">
            <asp:ListItem>1</asp:ListItem>
            <asp:ListItem>2</asp:ListItem>
            <asp:ListItem>3</asp:ListItem>
        </asp:RadioButtonList>
        <br />
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Label ID="Label1" runat="server"></asp:Label>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="RadioButtonList1" EventName="SelectedIndexChanged" />
            </Triggers>
        </asp:UpdatePanel>
    </form>