...
    ComboBox1: TComboBox; 
    procedure FormCreate(Sender: TObject); 
  private 
    { Private-Deklarationen } 
  public 
    procedure Dropped(Sender: TObject; Button: TMouseButton; 
      Shift: TShiftState; X, Y: Integer); 
  end; 
 
var 
  Form1: TForm1; 
 
implementation 
 
{$R *.DFM} 
 
type 
  TC = class(TCombobox); 
 
procedure TForm1.Dropped(Sender: TObject; Button: TMouseButton; 
  Shift: TShiftState; X, Y: Integer); 
begin 
  if not application.terminated then 
  // mach was, z.B.: 
  Beep; 
end; 
 
procedure TForm1.FormCreate(Sender: TObject); 
begin 
// --- zum Testen ---------------- 
  combobox1.items[0] := 'Zeile 1'; 
  combobox1.items[1] := 'Zeile 2'; 
  combobox1.items[2] := 'Zeile 3'; 
  combobox1.items[3] := 'Zeile 4'; 
  combobox1.items[4] := 'Zeile 5'; 
// ------------------------------- 
  combobox1.itemindex := 0; 
  TC(Combobox1).OnMouseDown := Dropped; 
end;