To study the metabolic effects of anesthesia, whole-body oxygen consumption (VO2) was compared on 242 occasions in six dogs under standard conditions while awake, sleeping, or anesthetized. The dogs were trained to lie unrestrained in the lateral position for the measurement of VO2(STPD) in the unanesthetized state. Arterial blood gas tensions,pH, heart rate, and blood pressure also were determined. The maximum VO2of the alert resting and the minimum of the drowsy resting state averaged (±SE) 5.57 ± 0.48 and 3.97 ± 0.41 ml · kg-1· min-1, respectively. VO2was lowest and least fluctuating during natural sleep (2.46 ± 0.2 ml · kg-1· min-1). During deep anesthesia with methohexital, thiopental, and etomidate, VO2averaged (±SE) 4.68 ± 0.26, 4.26 ± 0.28, and 4.77 ± 0.35 ml · kg-1· min-1during spontaneous ventilation (open-circuit flow-through technique) and 3.54 ± 0.27 ml · kg-1· min-1during controlled ventilation (open-circuit collection technique) with 2% halothane. Anesthesia reduces VO2relative to the resting values of the alert state but increases it relative to that of natural sleep. Accordingly, anesthetics should not be considered general metabolic depressants without qualification.