cascading dropdownlist

Question

  <asp:DropDownList ID="txtCity" runat="server" CssClass="input_bar" 

                            onselectedindexchanged="txtCity_SelectedIndexChanged" TabIndex="0">

                        </asp:DropDownList>

                       <ajaxToolkit:CascadingDropDown ID="AutoCompleteCity" runat="server" Category="Country"

                            TargetControlID="txtCity" PromptText="Hyderabad" ServicePath="WebService.asmx"

                            ServiceMethod="GetCityList">

                        </ajaxToolkit:CascadingDropDown>

 protected void txtCity_SelectedIndexChanged(object sender, EventArgs e)

    {

        Session["City"] = Server.HtmlEncode(txtCity.SelectedItem.Text);

        lblTitle.Text = txtCity.SelectedItem.Text;

    }
but the event is not raised y? please help me