Grapes (the fruit) are very high in sugars. While most of us crave sweets and enjoy them, it's best to limit them, especially in rabbits that have such delicate digestive systems. For some unknown reason (scientists still don't know the cause), grapes can cause kidney failure in dogs. My old dog (who lived to 14) loved grapes, when they were ripe, she'd go down the grape arbor and eat the low grapes. I understand wild dogs have also been seen eating grapes, but for other dogs, even one grape or raisin can be deadly. No one seems to know why. For rabbits, the real problem may be the excess sugar messing with the digestive system, it's easy to disrupt normal rabbit digestion, and hard to get it going properly again, so sweet treats of any kind are generally given very sparingly and not very often. I'm sure there are some rabbits that will have no problem with them, and others that will have immediate ill effects. Seems to be the way of things. So, to be on the safe side, we usually err on the side of caution.