please look this simple example
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
int i = e.Row.RowIndex;
DropDownList ddl = (DropDownList)e.Row.FindControl("ddl");
TextBox tx = (TextBox)e.Row.FindControl("txt");
if (ddl.SelectedValue == "DR".ToLower())
tx.Enabled = false;
else
tx.Enabled = true;
ddl.Attributes.Add("onchange", "if (" + ddl.ClientID + ".value=='dr'){" + tx.ClientID + ".disabled=true;}else{" + tx.ClientID + ".disabled=false;} ");
}
}