Aww, poor Winkie! Sounds like a testicular tumor . I whipped out my handy Rat Health Care book by Debbie Ducommun, but unfortunately wasn't able to find much about these types of tumors. I googled it, and here you go:

http://ratguide.com/health/neoplasia...lar_tumors.php

The recommended treatment is neutering him. Definately talk with your vet about what to do, and about treatment. I have two girlies and dread the day one of them gets a tumor, as about 50% of girls get tumors. Give the boy a neck scritch for me, ok?

~abigail