Preferred ambient temperature (T) and breathing rate were measured in free-moving mice exposed to 2,450-MHz microwaves. A waveguide-exposure system was imposed with a longitudinal temperature gradient that permitted mice to select their preferred T. Breathing rate was determined by analyzing the rhythmic shifts in microwave energy not absorbed by the animal. Without microwave exposure mice selected an average T of 31 C. This preferred T did not change until the specific absorption rate (SAR) at 2,450 MHz exceeded approximately 7.0 W. per kg. Mice maintained their breathing rate near base-line levels by selecting a cooler T during microwave exposure. In contrast, mice maintained at 31 C underwent a sharp increase in breathing rate when SAR exceeded approximately 7.0 W. per kg. Mice exposed to microwaves in a waveguide with a temperature gradient increased breathing rate 0.6 breaths/min per unit increase in SAR, whereas without the temperature gradient breathing rate increased by 9.6 breaths/min per unit increase in SAR. Data from this study support previous studies that have shown behavioral thermoregulation is more effective (or efficient) in minimizing a thermal load than autonomic thermoregulation.