Colonic temperature was measured in naive BALB/c mice and golden hamsters immediately following 90-min exposures to 2450-MHz radiofrequency (RF) radiation at an ambient temperature (Ta) of 32.2 or 35 degrees C (dry air). Exposures were performed in a temperature-controlled waveguide which permitted continuous monitoring of the specific absorption rate (SAR) of RF energy. At a Ta of 32.2 degrees C the threshold SAR for elevating colonic temperature and the SAR resulting in a 0.5 degree C elevation in colonic temperature were, respectively, 4.3 and 6.5 W/kg for the mouse and 0.68 and 1.1 W/kg for the hamster. At a Ta of 35 degrees C these values were 0.12 and 0.63 W/kg for the mouse and 0.46 and 0.8 W/kg for the hamster. The SARs required to elevate body temperature in the mouse and hamster at these relatively warm Ta's are considerably lower than those required at cooler Ta's of 20 to 30 degrees C. Overall, the hamster became hyperthermic at lower SARs than the mouse. Ta's of 35 degrees C and greater are frequently encountered during heat waves in the summer months. Under such stressful environmental conditions where heat loss is impaired, absorption of RF radiation at relatively low SARs may lead to significant hyperthermia which would otherwise be readily dissipated at lower Ta's.