From the May/June 1996 Rat & Mouse Gazette
I think Lori Chaddock did an excellent job in discussing the potential risks, or lack thereof, of feeding chocolate to rats, and especially in revealing the lack of available data. However, I would like to point out that there has been research done in rats studying the toxic effects of theobromine, which is the component of chocolate that causes the toxic effect seen in dogs.
At very high doses (500 mg/kg daily for 5 to 7 days) of theobromine, the ONLY effect on the rats' health that was noted was sterility in male rats due to damage to cells in the testes. No other cell populations appeared to be affected. (1). Translated, this means that even if your rat (assuming a weight of 330g) eats two entire small bars of Hershey's milk chocolate, he should be fine except for some possible effects on future fatherhood. Even pregnant rats fed high levels of cocoa powder (5 to 7.5% of their diet every day) showed no signs of toxicity, nor did their pups, although they were slightly smaller than normal. (2). In addition, in the world of research, some labs even entice rats that have lost their appetites by giving them chocolate chip cookies.
The research summarized above reveals that chocolate does not offer a serious health threat to rats as it does to dogs, even in very high amounts. In conclusion, I believe that the advice that Lori gave is very sound. As with any sugary treat, it is probably best to give chocolate in small and infrequent amounts. However, if your rat ever gets into some chocolate cache of yours and does ingest a lot of chocolate, it looks like it should be no cause for excessive worry.