O2 uptake was measured during light halothane anesthesia, controlled ventilation, and operation. In the absence of premedication and without thiopental, O2 uptake averaged 84% of predicted basal values in paralyzed patients and 100% in unparalyzed patients. The difference is attributed to increased O2 consumption of skeletal muscle in the unparalyzed state. With premedication and thio-pental, O2 uptake averaged 84% initially but increased with time. With premedication alone, O2 uptake averaged 88% initially and increased to 94% in 2 to 5 hours. The return of O2 uptake to basal values in both groups is attributed primarily to diminution of the effect of the previously administered drugs. Significant effects of halothane concentration per se on O2 uptake were not observed. An association between O2 uptake and esophageal temperature was demonstrated only in paralyzed patients.