<script type="text/javascript" language="javascript">
        function DDLBind()
        {
            var id= document.getElementById("ddlFirst").options.value;
            DropDownBind.bindDDLModel(id,DDL);
        }
    </script>
    <script type="text/javascript" language="javascript">
    <!--
        function DDL(response)
        {
            var ds=response.value;
            var obj=document.getElementById("ddlSecond");
            
            while(obj.options.length)
            {
                obj.remove(0);
            }
            
            obj.options[0]=new Option("Please wait....","0");
            
            if(ds != null && typeof(ds) == "object" && ds.Tables != null)
            { 
                obj.options[0]=new Option("-- Select --","0");
                for(var i=0,j=1;i                {
                    //alert(ds.Tables[0].Rows.length);
                    obj.options[j]=new Option(ds.Tables[0].Rows[i].Model,ds.Tables[0].Rows[i].ModelId);
                }
            }
            else
            {
                obj.options[0]=new Option("Please try again");
            }
        }
        -->
    </script>



<table width="100%">
            <tr>
                <td>First DropDownList :</td>
                <td><asp:DropDownList ID="ddlFirst" runat="server" onchange="javascript:return DDLBind();" /></td>
            </tr>
            <tr>
                <td>Second DropDownList :</td>
                <td><asp:DropDownList ID="ddlSecond" runat="server" ></td>
            </tr>
        </table>

public partial class DropDownBind : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection("server=server;database=click2ins;uid=sa;pwd=");
    SqlTransaction Tran;
    protected void Page_Load(object sender, EventArgs e)
    {
        Ajax.Utility.RegisterTypeForAjax(typeof(DropDownBind));
        try
        {
            con.Open();
            Tran = con.BeginTransaction(IsolationLevel.ReadCommitted);
            SqlCommand cmd = new SqlCommand("select MakeId,MakeName from ms_make", con, Tran);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds, "Make");
            ddlFirst.DataSource = ds.Tables["Make"];
            ddlFirst.DataValueField = "MakeId";
            ddlFirst.DataTextField = "MakeName";
            ddlFirst.DataBind();
            Tran.Commit();
        }
        catch
        {
            Tran.Rollback();
        }
        finally
        {
            con.Close();
        }
    }

    [Ajax.AjaxMethod()]
    public DataSet bindDDLModel(string id)
    {
        DataSet ds = new DataSet();
        try
        {
            con.Open();
            Tran = con.BeginTransaction(IsolationLevel.ReadCommitted);
            string query = "select ModelId,Model from ms_model where MakeId='" + id + "'";
            SqlCommand cmd = new SqlCommand(query, con, Tran);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            
            da.Fill(ds, "Model");
            Tran.Commit();
        }
        catch
        {
            Tran.Rollback();
        }
        finally
        {
            con.Close();
        }
        return ds;
    }
}