Although suppression of thermoregulatory mechanisms by anesthetics is generally assumed, the extent to which thermoregulation is active during general anesthesia is not known. The only thermo-regulatory responses available to anesthetized, hypothermic patients are vasoconstriction and non-shivering thermogenesis. To test anesthetic effects on thermoregulation, the authors measured skin-surface temperature gradients (forearm temperature - fingertip temperature) as an index of cutaneous vasoconstriction in unpremedicated patients anesthetized with 1% halothane and paralyzed with vecuronium during elective, donor nephrectomy. Patients were randomly assigned to undergo maximal warming (warm room, humidified respiratory gases, and warm intravenous fluids; n = 5) or standard temperature management (no special warming measures; n = 5). Skin-surface temperature gradients ≥ 4° C were prospectively defined as significant vasoconstriction. Normothermic patients [average minimum esophageal temperature = 36.4 ± 0.3° C (SD)] did not demonstrate significant vasoconstriction. However, each hypothermic patient displayed significant vasoconstriction at esophageal temperatures ranging from 34.0 to 34.8° C (average temperature = 34.4 ± 0.2° C). These data indicate that active thermoregulation occurs during halothane anesthesia, but that it does not occur until core temperature is ±2.5° C lower than normal. In two additional hypothermic patients, increased skin-temperature gradients correlated with decreased perfusion as measured by a laser Doppler technique. Measuring skin-surface temperature gradients is a simple, non-invasive, and quantitative method of determining the thermoregulatory threshold during anesthesia.