Six male New Zealand white rabbits were individually exposed to 600 MHz radiofrequency (RF) radiation for 90 min in a waveguide exposure system at an ambient temperature (T_a) of 20 or 30°C. Immediately after exposure, the rabbit was removed from the exposure chamber and its colonic and ear skin temperatures were quickly measured. The whole-body specific absorption rate (SAR) required to increase colonic and ear skin temperature was determined. At a T_a of 20°C the threshold SAR for elevating colonic and ear skin temperature was 0·64 and 0·26 W/kg, respectively. At a T_a of 30°C the threshold SARs were slightly less than at 20°C, with values of 0·26 W/kg for elevating colonic temperature and 0·19 W/kg for elevating ear skin temperature. The relationship between heat load and elevation in deep body temperature shown in this study at 600 MHz is similar to past studies which employed much higher frequencies of RF radiation (2450-2884 MHz). On the other hand, comparison of these data with studies on exercise-induced heat production and thermoregulation in the rabbit suggest that the relationship between heat gain and elevation in body temperature in exercise and from exposure to RF radiation may differ considerably. When combined with other studies, it was shown that the logarithm of the SAR required for a 1·0°C elevation in deep body temperature of the rabbit, rat, hamster, and mouse was inversely related to the logarithm of body mass. The results of this study are consistent with the conclusion that body mass strongly influences thermoregulatory sensitivity of the aforementioned laboratory mammals during exposure to RF radiation.