Male Sprague-Dawley rats were maintained at an ambient temperature (Ta) of 10 C and exposed to 600 MHz radiofrequency (RF) radiation while metabolic rate (MR) was measured by indirect calorimetry. RF radiation exposures were made in a waveguide-type system which permitted the continuous control of specific absorption rate (SAR). SAR's of 1 to 5 W/kg led to significant reductions in MR when averaged 30 to 60 min after the initiation of RF radiation exposure. The total decrease in MR during RF radiation exposure accounted for approximately 37% of the total RF heat load. Treatment of the rats to the same SARs at Ta of 10 C resulted in a significant elevation of colonic temperature. Thus, in spite of the decrease in MR, heat gain still exceeded heat loss during RF radiation exposure with a resultant elevation in deep body temperature.