Whether living outdoors or indoors, rabbits are heat-sensitive. If the temperature begins to creep over 80 degrees Fahrenheit, you should take extra precautions to prevent heat stroke and death in rabbits. If you work away from home during the day, make sure someone comes home midday to cool your rabbit down, provide shade, turn on fans if indoors, and place several 2-liter bottles of frozen water in his cage, tightly capped. More frequent grooming in the summer can remove excess hair and help keep your rabbit cool.
For more information, please read Summer Rabbit Care