The ability of rats to regulate body temperature in a cool helium-oxygen (heliox) atmosphere at high pressure was studied. Thermoregulatory ability in seven awake rats was assessed in a pressure chamber at 1.5, 21 and 41 ATA by monitoring body temperature and oxygen consumption during a gradual reduction of ambient temperature. Body temperature was measured by a radiotelemetry transmitter implanted into the intraperitoneal cavity. The thermal conductance of the rats increased from 0.37 to 0.75 W kg-1 degrees C-1 as air was substituted by heliox and increased further to 1.47 and 1.81 W kg-1 degrees C-1 as the pressure was increased to 21 and 41 ATA, respectively. At high pressure the rats were able to maintain body temperature until the metabolic rate reached a value of 21.9 +/- 2.0 W kg-1, which corresponds to peak metabolic rate. This occurred at an ambient temperature of approximately 21 degrees C. Below this ambient temperature the rats did not manage to maintain body temperature. The results show that the thermal effector mechanisms of rats respond adequately to a slowly decreasing ambient temperature in hyperbaric heliox environments.