The scenario tends to be similar to dogs in a car. Rabbits and Guinea pigs are often left in a confined area where they are unable to control their own temperature – typically a run left in full sunlight with inadequate shade, or a hutch with poor ventilation on a hot day. Having no water to drink as well will exacerbate the problem. The other cause can be a rabbit suffering from epilepsy – a long fit on a warm day will increase an animal’s temperature markedly, though fortunately this is quite rare.
Signs of heat stroke are often not very specific. An effected individual will tend to be panting and puffing, be very quiet and may even be collapsed flat out. The pet will obviously feel very hot, especially on its ears and head. If your pet is obviously distressed and has been in the sun, it is important to cool them down as soon as possible, as they can die quite quickly from heat stroke.
Place your rabbit or guinea pig in a cool area, and wrap him in a cold wet towel. Use a fan or a cool breeze if available as well. It is usually best to get your pet checked at a vets fairly promptly, as it may need fluids and anti-inflammatories to help with the shock and dehydration. If the problem was caused by fitting then this will obviously need to be investigated and treated as well.