Hi EveryOne

Here is the newer version of the problem i discussed and was solved with all your help.

I now have 3 DropDownLists. Now the idea is the same I populated the first DropDownList on PageLoad. Then OnSelectedIndexChanged i popolated the second one. Then on selection on the second one i populated the third one.

[PROBLEM] The problem is the same that the second DropDownList is now doubling and trippling the seleted items. I mean it concatenates the new items with the old ones, even the searching criteria is changed. I tries DropDownList7.Items.Clear(), but it is not working.

void Page_Load(Object s,EventArgs e)
{
  objConn = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath("/") + "aeg.mdb");
  objConn.Open();
  objCmd = new OleDbCommand("SELECT DISTINCT Country from Institutes",objConn);
  objReader = objCmd.ExecuteReader();

if(! IsPostBack)
  while(objReader.Read())
   {
   DropDownList6.Items.Add(Convert.ToString(objReader["Country"]));
   }
}

Then on Selected Index change:

void DropDownList6_SelectedIndexChanged(object sender, EventArgs e) {

 objCmd = new OleDbCommand("SELECT DISTINCT City from Institutes where Country = '"+ DropDownList6.SelectedItem.Text + "'",objConn);
objReader = objCmd.ExecuteReader();

  while(objReader.Read())
   {
  DropDownList7.Items.Add(Convert.ToString(objReader["City"]));

    }

}

          void DropDownList7_SelectedIndexChanged(object sender, EventArgs e) {
    
               objCmd = new OleDbCommand("SELECT DISTINCT Institute from Institutes where Country = '"+ DropDownList6.SelectedItem.Text + "' and City = '" + DropDownList7.SelectedItem.Text + "'",objConn);
               objReader = objCmd.ExecuteReader();
    
               while(objReader.Read())
               {
                 DropDownList8.Items.Add(Convert.ToString(objReader["Institute"]));
    
               }
    
         }

        void DropDownList6_SelectedIndexChanged(object sender, EventArgs e)
        {

            objCmd = new OleDbCommand("SELECT DISTINCT City from Institutes where Country = '" + DropDownList6.SelectedItem.Text + "'", objConn);
            objReader = objCmd.ExecuteReader();

            DropDownList7.Items.Clear();
            while (objReader.Read())
            {
                DropDownList7.Items.Add(Convert.ToString(objReader["City"]));

            }

        }

        void DropDownList7_SelectedIndexChanged(object sender, EventArgs e)
        {

            objCmd = new OleDbCommand("SELECT DISTINCT Institute from Institutes where Country = '" + DropDownList6.SelectedItem.Text + "' and City = '" + DropDownList7.SelectedItem.Text + "'", objConn);
            objReader = objCmd.ExecuteReader();

            DropDownList8.Items.Clear();
            while (objReader.Read())
            {
                DropDownList8.Items.Add(Convert.ToString(objReader["Institute"]));

            }

        }