Most rabbits will love grapes because they taste sweet and are full of sugar. However if you feed your rabbits grapes you should be careful to not feed them too much because it can easily cause digestive troubles and diarrhea. This happens when bad bacteria quickly digest the sugar and produce gas (your rabbit will become bloated) and change the pH of the digestive system. This reduces the growth of good bacteria that normally produce the B-vitamins and other important nutrients that your rabbit needs to be healthy and active.
The main diet of a a rabbit should be properly formulated and well balanced pellets that contain everything your rabbit needs to be healthy. They should be made primarily from hay and have a high level of fiber. For adult rabbits this diet should also be low in carbohydrates (sugar and starch). It is best to feed adult rabbits a low-energy grain and soy-free rabbit food rich in timothy hay (and flax seeds for the omega 3's). Baby rabbits need a high energy diet and are healthier when eating a diet that is high in fat "like mamma's milk." For long-term health it is also very important to get a good start by feeding them a diet that is rich in specific minerals and provides enough high quality balanced protein.
Following these feeding recommendations will keep your pet rabbit healthy. So can rabbits eat grapes? Yes they can but only as a treat!