Rabbits can consume up to two tablespoons worth of grapes without becoming ill. While grapes are not particularly toxic, their negative health effects on rabbits are similar to those of other sugary foods, including carrots. A rabbit's digestive tract relies upon large amounts of fiber to keep it functioning, and sugary foods can interfere with their delicate digestive balance.
Contrary to popular belief, a rabbit's diet primarily consists of fibrous grass and hay, not fruits or root vegetables. If the rabbit is allowed to consume grapes and other sugary foods in large amounts, harmful bacteria can build up in the intestines and cause illness. In addition, a diet high in sugary fruit and low in fiber can cause gastrointestinal stasis, a potentially fatal condition in which the digestive tract shuts down.