A normal body temperature for a Jack Russell Terrier is 101 to 102.5 degrees Fahrenheit, or 38.3 to 39.2 degrees Celcius, according to guidelines from the American Kennel Club. The normal body temperature range is the same for all dogs regardless of size, as is true of normal human temperature.
In order to determine a dog's temperature, the owner or veterinarian needs to use a rectal thermometer. Canine ear canals cannot provide an accurate temperature read due to their shape, hairiness and dirt build-up, states the American Kennel Club. The thermometer should be inserted no more than 2 inches into the rectum with petroleum jelly.