BACKGROUND Hypercapnia, which may be encountered during diving operations or by patients under general anesthesia with spontaneous respiration, alters the basic thermoregulatory responses of animals and humans. In cold-exposed animals, 3-10% inspired CO2 impairs thermal homeostasis by attenuating shivering and promoting heat loss through peripheral vasodilation. Experimental results with humans are equivocal. OBJECTIVE To determine the effects of hypercapnia on warm and cold thermoregulatory responses and core cooling rates during mild cold stress, we examined the core temperature (TCO) thresholds for sweating, vasoconstriction and shivering, as well as core cooling rates of eight subjects immersed in 28 degrees C water under hypercapnic conditions. METHODS On two separate days, subjects exercised on an underwater cycle ergometer to elevate TCO above the sweating threshold. They then rested and cooled until they shivered vigorously. Subjects inspired humidified room air during the control trial and 4% CO2/20.9% O2/balance N2 during the hypercapnia trial. RESULTS Hypercapnia lowered the threshold for shivering by 0.13 degrees C and increased the core cooling rate by 0.35 degrees C.h-1 (25%). Minute ventilation was approximately 12.0 L.min-1 throughout cooling during control and increased from 25.3 to 28.7 L.min-1 during hypercapnia. CONCLUSIONS Therefore, hypercapnia enhances the core cooling rate during mild cold stress. This may be attributed in part to a delay in shivering onset, as well as increased respiratory heat loss during hypercapnic hyperventilation.