This paper deals with the development of thermoregulation of Mongolian gerbil (Meriones unguiculatus). The body temperatures and oxygen consumptions of 28 litters totalled 157 young were measured at ambient temperatures of 10°-35℃.After birth the homeothermic ability increases gradually, and reaches that of adult's at 31 days old. Even at 1 day old, the young response to cold by increasing oxygen consumption. This shows that the chemical thermoregulation is already present at this time. By about 20 days old...