Bob
In Access, the things that hold buckets o' data are called 'tables', not
'databases'. The entire .mdb file is called a 'database' in Access. I
offer these definitions so you can be assured that what you are asking for
and what folks are answering match up better.
One approach to doing what you've described would be to change the source of
the subform (by the way, another definition for Access -- a subform is a
form that is embedded within a "main form").
Thus, subforms do not "pop up". If you want a form to pop up, it's called
either a "popup form" or a "dialog box").
To get a form to pop up, use code in the combo box's AfterUpdate event that
opens the new form.
To get a different subform to "load" on your main form, again add code to
the combo box's AfterUpdate event, but this time, in the code, alter the
'source' for the subform.
Regards
Jeff Boyce
Microsoft Office/Access MVP