Dropdown Method [Access 2003 VBA Language Reference]
You can use the Dropdown method to force the list in the specified combo box to drop down.
expression Required. An expression that returns one of the objects in the Applies To list.
For example, you can use this method to cause a combo box listing vendor codes to drop down when the vendor code control receives the focus during data entry.
If the specified combo box control doesn't have the focus, an error occurs. The use of this method is identical to pressing the F4 key when the control has the focus.
The following example shows how you can use the Dropdown method within the GotFocus event procedure to force a combo box named SupplierID to drop down when it receives the focus.
Private Sub SupplierID_GotFocus() Me!SupplierID.Dropdown End Sub