How to filter a dropdownlist in Django's admin when a selection is made on another dropdownlist

I have two dropdownlists in a Django admin site. For example, I have SelectCountry and SelectRegion. Region has a foreignkey relationship to Country. How do I ensure that when a Country is selected, the Regions are filtered based on that Country?

N.B: I am using django-grappelli for my admin backend as well.

Any ideas will be appreciated. Thanks.