The effects of anesthetic depth on cerebral venous Po2 (Pvo2)and cerebral arteriovenous oxygen content difference (A-V)o2 were studied in three groups of six dogs each. Normal arterial Pco2, Po2, and blood pressure were maintained throughout. Group I animals were studied at 0.1, 0.6, 1.3, 1.8, and 2.5 MAC halothane. Mean Pvo2's were 37.0, 43.4, 48.5, 58.0, and 71.7 torr, respectively, and mean (A-V)o2's were 9.8, 7.6, 6.5, 4.2, and 3.2 ml/100 ml, respectively. Group II animals were studied during anesthesia with 0.7, 1.4, and 2.1 MAC halothane; 0.7, 1.4, and 2.1 MAC enflurane; and three mixtures of halothane and enflurane with total MAC's 1.4 or 2.1. Group III dogs were given 0.7, 1.4, and 2.1 MAC halothane and the same MAC multiples of Forane (isoflurane). Pvo2 and (A-V)o2values for Groups II and III were the same at each value of MAC, regardless of which anesthetic agent or mixture was given.