Access 2000 field auto populate question

I need to have a field in a form auto populate based on input to a previous fields entry.. example,, i have a combo box for a part number with the row source entry coming from a query,, when this field has the part number selected and you tab out of it i would like to have the next field auto populate with the description.. anybody have any clues.. if its vba you will have to be “do a, b, c, etc” with your explaination.. thanks..