I assume that you mean a Combo Box and not a List Box and
that it is unbound. If it were bound, it would
automatically show the value in the first record.
You can have it take on the value of a row in the dropdown
list by trying the following code for the OnEnter event of
the Combo Box:
Me![comboboxname].Dropdown
Me![comboboxname] = Me![comboboxname].Column(0,0)
In this case, the first value in the list would become the
default. You could change the second index value of the
Column property to access other row values.