To evaluate physiologic responses to mild perianesthetic hypothermia, we measured tympanic membrane and skin-surface temperatures, peripheral vasoconstriction, thermal comfort, and muscular activity in nine healthy male volunteers. Each volunteer participated on three separate days: 1) normothermic isoflurane anesthesia; 2) hypothermic isoflurane anesthesia (1.5 degrees C decrease in central temperature); and 3) hypothermia alone (1.5 degrees C decrease in central temperature) induced by iced saline infusion. Involuntary postanesthetic muscular activity was considered thermoregulatory when preceded by central hypothermia and peripheral cutaneous vasoconstriction. Tremor was considered normal shivering when electromyographic patterns matched those produced by cold exposure in unanesthetized individuals. During postanesthetic recovery, central temperatures in hypothermic volunteers increased rapidly when residual end-tidal isoflurane concentrations were less than or equal to 0.3% but remained 0.5 degree C less than control values throughout 2 h of recovery. All volunteers were vasodilated during isoflurane administration. Peripheral vasoconstriction occurred only during recovery from hypothermic anesthesia, at end-tidal isoflurane concentrations of less than approximately 0.4%. Spontaneous tremor was always preceded by central hypothermia and peripheral vasoconstriction, indicating that muscular activity was thermoregulatory. Maximum tremor intensity during recovery from hypothermic anesthesia occurred when residual end-tidal isoflurane concentrations were less than or equal to 0.4%. Three patterns of postanesthetic muscular activity were identified. The first was a tonic stiffening that occurred in some normothermic and hypothermic volunteers when end-tidal isoflurane concentrations were approximately 0.4-0.2%. This activity appeared to be largely a direct, non-temperature-dependent effect of isoflurane anesthesia. In conjunction with lower residual anesthetic concentrations, stiffening was followed by a synchronous, tonic waxing-and-waning pattern and spontaneous electromyographic clonus, both of which were thermoregulatory. Tonic waxing-and-waning was by far the most common pattern and resembled that produced by cold-induced shivering in unanesthetized volunteers; it appears to be thermoregulatory shivering triggered by hypothermia. Spontaneous clonus resembled flexion-induced clonus and pathologic clonus and did not occur during hypothermia alone; it may represent abnormal shivering or an anesthetic-induced modification of normal shivering. We conclude that among the three patterns of muscular activity, only the synchronous, tonic waxing-and-waning pattern can be attributed to normal thermoregulatory shivering.