Some dogs really like grapes and raisins and will even eat them directly from the vine. However it has been known for a while that some dogs will go on to develop acute renal (kidney) failure shortly after eating these foods.
It is currently unknown why these fruits are toxic to dogs. It may not be the fruit itself, but rather a mycotoxin (from a mould or fungus on the grapes), or perhaps a chemical with which the grapes have been sprayed such as a pesticide or herbicide which causes the problem.
If a dog eats a quantity of raisins or grapes then there is a possibility that they may develop signs of toxicity. Such signs include vomiting, diarrhoea and excitable or nervous behaviour; usually within 24 hours of consumption. Dogs may then develop other problems such as abdominal pain, lethargy, anorexia and depression. Acute renal failure may occur and the dog may die.
Vets may be able to treat this problem by making the animal vomit if the grapes or raisins were fairly recently eaten and have not yet passed through the stomach (within 2 hours of consumption). Following this the administration of activated charcoal can help to absorb toxic chemicals in the gut before they can cause a problem.
Since the main problem associated with the ingestion of grapes and raisins is acute renal failure it is important to support the kidneys and this can be done by giving the animal intravenous fluids. Dogs may need i/v fluids for around 48 hours or longer and the vet will be able to monitor your pet's kidney function via blood tests and monitoring urine output etc during the treatment.
I would not be at all surprised if one day we find that this toxicity is due to a mycotoxin or chemical spray and nothing to do with the grapes themselves. However since there is no way to tell at the moment it is wise not to allow your dog to eat either raisins or grapes. If you suspect that your dog has eaten grapes or raisins and may be showing signs of toxicity consult your vet urgently.