In addition to "rat" or "lab" blocks (basically square kibble), you can supplement your rats diet with fresh fruits, vegetables, (my rats are happy to share whatever I'm having as a vegetable side-dish for dinner) and non-sweetened cereals such as Cheerios. Rats can eat seeds and grains, but shouldn't have them as the bulk of their diet. They can have occasional treats like a tiny piece of chocolate (it's not toxic to rats) or a yogurt drop. Also make sure your rats have fresh water at all times.

Here are some links that provide a lot of good suggestions for feeding your rat as well as receipes for preparing a homemade diet:

<http://hometown.aol.com/RistoRat/nutrition.html> <http://www.geocities.com/suebee277/rat/diet.html> <http://www.rmca.org/Articles/recipes1.html> <http://kuddlykorner4u.com/RatDiet.html>