how to bind one dropdownlist to other drpdown list

Make First Dropdown box Autopost Back Property True:

 private void FillDropDownList1()
    {

        DataSet ds = new DataSet();
        SqlDataAdapter myda = new SqlDataAdapter("select field1 from tablename",connection;
        myda.Fill(ds);
        dr1.items.clear(); //added
        dr1.DataSource = ds;
        dr1.DataTextField = "field1"; //added
        dr1.DataValueField = "field1";
        dr1.DataBind();

    }

put this on Page load:
        if (!IsPostBack)
        {
             FillDropDownList1();
        }

now, double click on first dropdown:

    protected void dr1_SelectedIndexChanged(object sender, EventArgs e)
    {
              DataSet ds = new DataSet();
        SqlDataAdapter myda = new SqlDataAdapter("select field1 from tablename where column1='"+dr1.selectedItem.value+"'",connection;
        myda.Fill(ds);
        dr2.items.clear(); //added
        dr2.DataSource = ds;
        dr2.DataTextField = "field1"; //added
        dr2.DataValueField = "field1";
        dr2.DataBind();

    }

this will surely help you. otherwise post it.