Sexing Rats: Is This Rat a Boy or a Girl? - RattyRat
RattyRat.com was a great website with vast information for pet rat lovers. Unfortunately it has closed down, but our collaboration will remain and JoinRats will host some of that site's useful information. Here are a few sharp photographs on the sex of baby rats.
Careful: Separate baby boy and girl rats from each other, and the boys from their mother, by age 5 weeks, or the females will become pregnant and you'll start all over again!