If you have a bunny outside in a hutch, make sure your hutch is in a shaded area to keep cool, if not move it to a shaded area.
Also even though it is in the shaded area, also included a Frozen Water Bottle you would pick up at the store or full up a old soda bottle with water and freeze it. Place it each morning in the cage with you bunny so they can lay near it too keep cool.
It is important to check on your bunny throughout the day to make sure they are cool and their drinking water is filled up at all times.
Inside bunnies are keep them cool with a AC cooling your house or have a fan near them to keep cool!
I will do a additional post on how to tell if your bunny has heat stroke and what to do.
Here are the symptoms of heat stroke in rabbits:
•Reddening of the ears.
What should you do if your rabbit shows signs of heatstroke?
Your first goal will be to relocate your bunny to a cool place away from any sun. Dampen the ears with cool (not cold) water as this will help to bring down his/her body temperature. Rabbit’s ears are his/her air conditioner/ that is where they regulate their body temperature. Give your bunny plenty of fresh, cold water with a few ice cubes in it and call your rabbit savvy vet for further instructions.
Also MOST IMPORTANT it also good to contact your vet as well,
This is a GREAT article on Heat Stroke on Rabbits and what to do and it also shows a video of a rabbit that has it.