BACKGROUND Typically, core temperature rapidly decreases after induction of anesthesia, but reaches a stable plateau after several hours. This plateau typically occurs in conjunction with the onset of thermoregulatory vasoconstriction. Decreased heat loss, caused by vasoconstriction, may not be sufficient to establish thermal steady state without a concomitant increase in heat production. Accordingly, the authors tested the hypothesis that nonshivering thermogenesis contributes to thermal steady state during anesthesia. Rewarming from hypothermia is often associated with an afterdrop (a further reduction in core temperature, despite cutaneous warming). Because total body heat content increases during cutaneous warming, heat storage during afterdrop must reflect increased temperature and heat content of the peripheral tissue mass. Thermal balance was measured during rewarming to estimate the thermal capacity of the peripheral tissues. METHODS Five volunteers were anesthetized with isoflurane and paralyzed with vecuronium. Oxygen consumption was measured during cooling to a core temperature at least 1 degree C less than that which triggered vasoconstriction. Volunteers were subsequently rewarmed using a circulating-water blanket and forced-air warmer. Oxygen consumption and cutaneous heat flux were measured to assess thermal balance and peripheral tissue heat storage during rewarming. RESULTS The core temperature threshold for vasoconstriction was 35.2 +/- 0.8 degrees C. Oxygen consumption decreased 9 +/- 5%/degrees C during active cooling before vasoconstriction and 9 +/- 3%/degrees C after vasoconstriction. After the start of rewarming, core temperature continued to decrease for an additional 32 +/- 8 min. The magnitude of this afterdrop was 0.6 +/- 0.1 degree C. Peripheral tissue heat storage measured from the start of rewarming until the first net increase in core temperature was 144 +/- 60 kcal, which approximately equals 2 h of resting metabolic heat production. CONCLUSIONS The authors concluded that nonshivering thermogenesis is not an important thermoregulatory response in adults anesthetized with isoflurane. Afterdrop and delayed core temperature recovery during rewarming reflect the large heat storage capacity of peripheral tissues.