Although heating rate is important for stimulating thermoregulatory reflexes, it is not known if the control system differentiates between total heat gain and rate of heat gain. Exposing animals to microwaves inside a waveguide permits continuous monitoring of whole-body heat absorption. The data indicate that heat loss from the tail increases with the whole-body heat load accrued from microwave exposure. When heat loss is averaged with repect to heat load, the rate of heat absorption and ambient temperature increase the sensitivity of thermoregulatory centers that control peripheral heat loss from the tail of mice.