The purpose of this study was to determine the threshold specific absorption rate (SAR) during exposure to 2450 MHz continuous wave (CW) microwaves that affected thermoregulatory behavior in mice. A plexiglas shuttle box was placed inside a waveguide imposed with a temperature gradient. The temperature gradient allowed the mice to select a particular section of the shuttle box which was, presumably, related to their state of thermal comfort. Exposing the mice to 2450 MHZ inside the waveguide at SARs of 0-5.3 W kg for 1 hr caused no significant change in their preferred ambient temperature. Increasing SAR from 5.3 to 18.1 kg caused the animals to shift their position to the cooler end of the shuttle box. Following termination of microwave exposure animals that had selected a cool ambient temperature returned to the warm side of the shuttle box. It is concluded that for mice exposed to radiation at 2450 MHz the thermoregulatory behavior is significantly affected at SARs of 5.3 to 9.9 W kg.