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 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 C the threshold SAR for elevating colonic temperature and the SAR resulting in a 1.0 C elevation in colonic temperature were, respectively, 4.3 and 10.0 W/kg for the mouse and 0.69 and 1.9 W/kg for the hamster. At a Ta of 35 C these values were 0.12 and 5.3 W/kg for the mouse and 0.46 and 1.4 W/kg for the hamster. The SARs required to induce hyperthermia in the mouse and hamster at these relatively warm Ta's are considerably lower than those required at cooler Ta's of 20 to 3 C. Overall, the hamster became hyperthermic at lower SARs than in the mouse. Ta's of 35 C and greater are frequently encountered during heat waves in the summer months.