SARs of 2-5 W/kg led to significant reductions in metabolic rate when averaged from 30 to 60 min after the initiation of exposure. The total decrease in metabolic rate during exposure accounted for approximately 37% of the total radiofrequency heat load. Exposure of another group of animals to the same SARs at a ambient temperature of 10°C resulted in a significant elevation in colonic temperature. Thus, despite the decrease in metabolic rate, heat gain still exceeded heat loss during exposure, with a resultant elevation in deep body temperature. In conclusion, in a cold environment the ratexposed to radiofrequencyirradiation decreases its metabolic rate. However, the response time and efficiency is not adequate to prevent an increase in body temperature.