MSAccess: Limiting items in a dropdown.

I’m creating a little Access app and am having trouble. I have a form that writes to an table holding info about ads running in newspapers. It has two fields, Pub and AdSize and each of these is linked to tables holding the Pub names and AdSizes.I want the AdSize dropdown to be limited to the adsizes for the particular Pub selected from the Pub dropdown. I tried a query on the AdSize dropdown properties, with criteria of Pub equal to forms!tblPub!Pub, but the query prompts me for a parameter value instead of evaluating forms@tblPub!Pub.