The increased metabolic and respiratory demand during naloxone recovery from opioid-based anesthesia could be related to the return of thermoregulation in hypothermic patients and thus be avoided by preventing intraoperative hypothermia. In this study, we measured O2 uptake (Vo2) during naloxone-induced recovery in two groups of patients to determine the effect of intraoperative heat loss on postoperative Vo2 changes. In seven patients, intraoperative hypothermia was prevented (normothermic group), whereas hypothermia was allowed to develop in seven other patients (hypothermic group). Core and skin temperatures were measured throughout the study to calculate changes in body heat content. Before naloxone antagonism of fentanyl-supplemented anesthesia, core temperature (mean ± SEM) was 36.8 ± 0.1° C in the normothermic group and 34.2 ± 0.2° C in the hypothermic group (P < 0.001). After titrated administration of naloxone during recovery, Vo2 and minute ventilation (VE) increased in the hypothermic group, by 114 ± 37% and 97 ± 52% respectively (P < 0.05), with a three-fold increase in four patients. In the normothermic group, Vo2 increased significantly less (25 ± 5%), without any significant change in VE. The change in Vo2 and VE was significantly greater in patients who were hypothermic. Vo2 was integrated throughout the recovery period to calculate recovery energy expenditure. Recovery energy expenditure and intraoperative heat loss were highly correlated (r = 0.88; P < 0.01). This study demonstrates that the metabolic and respiratory stresses associated with naloxone-induced recovery from opioid-based anesthesia depend on the intraoperative heat loss and can therefore be reduced by preventing intraoperative hypothermia.