If I have a gridview with edit buttons, how do I show dropdowns inedit mode?

I have a gridview that allows the user to edit rows by clicking on an
EDIT button for each row. I notice that in EDIT mode I get checkboxes
for some fields, and textboxes for others. But I would like to have a
dropdownlist in some cases. I would have to prefill that dropdown
list with allowed values.

How do I do that?

Thanks,
Marvin