Normal body temperature for a guinea pig is between 99 and 103 degrees Fahrenheit, with an average rectal temperature of 103 F, according to LSU's School of Veterinary Medicine. Guinea pigs do best at an ambient temperature similar to comfortable room temperature for humans, about 70 degrees Fahrenheit.
Guinea pigs are vulnerable to heatstroke and tolerate lower temperatures better than higher temperatures. Outside temperatures above 80 degrees are risky, even if the animal is confined to the shade. Because they are so vulnerable to overheating, guinea pig cages should be kept out of direct sunlight and should be located in an area with good air circulation.