The first thing which should be done year round is to make sure the rabbits are kept in the shade. A temperature can change from anywhere from 10-40 degrees from the shade to the direct sun. The exception to this rule is during the winter, if your rabbit has the option of sitting in the sunlight when it’s cold (under freezing) that’s okay. If they get hot they will move if they have the option to.
Set up fans to get air moving in the barn. Hot air moving won’t make anyone feel better but it will keep vapors down and help to prevent disease. All year round I recommend at least an exhaust fan for the barn but in the summer if no A/C is available use fans to bring in fresh air.
You can freeze old coke bottles with water, and give it to them to lay up against and lick when it’s really hot. They usually don’t freeze over night after I used them, so I had to keep a couple extra ones for the ones that weren’t frozen. When I had about 30 rabbits, I used 50 bottles just to keep everyone cool all the time, rotating the bottles halfway into the day or whenever they had melted. Two liter bottles will last longer so you can use less.
You can set up a misting system, which I experimented with the final 2 years I needed it before my new barn was built. This works great but does wet everything in the barn. I only recommend using this in a pole barn and to keep anything that would be ruined if wet, well away. When used with fans this is an extremely effective way to keep bunnies cool.
If you don’t want to set up a misting system, you can manually mist rabbits with a spray bottle. Keep the mist on the ears only and turn the fans up. This should be done at least once an hour in extreme heat for the best results.
Babies who can hop around and have begun to take in pellets fair much better in the cold. If you’re overly concerned stuff the cage with hay or straw to nestle in. This will need to be cleaned out once a day from being soiled.
If you do use a heater be sure to keep it away from anything flammable and to keep it extra clean of fur, dust and cobwebs which could cause a spark and lead to a fire.
In the early stages of heat stroke, the rabbit will lay in its cage, stretched out, breathing heavy. They will often tilt their up upward and breath heavy. As heat stroke gets worse, they will become lazy and unresponsive. Sometimes they will turn blue around the nose. When this happens, they are very close to death.
If this still doesn’t work, fill up the sink and place him in room temperature water for a couple of seconds, careful to only submerge the body and not the head. Take him out and dry him off as best as you can, wrap the body in a towel tightly to dry him. If dehydration has happened in addition, you can put Gatorade in the water to provide electrolytes.