The effect of Althesin, an anesthetic comprising 2 steroids, on regional cerebral function was determined by measurement of regional cerebral glucose utilization. Rats were anesthetized with an i.v. dose of 4, 8 or 20 mg total steroid/kg. These doses produced anesthesia for 12, 18 and 37 min, respectively. There were no physiologically significant effects of Althesin (20 mg/kg) on body temperature, blood pH or blood gases. Blood pressure and heart rate decreased slightly after administration of Althesin. Althesin had a profound effect on glucose consumption in many, but not all, cerebral structures. The forebrain (especially cerebral cortex) was affected most, while the hindbrain was much less so or not at all. This pattern of functional depression is in accord with the minimal effects observed on physiologic variables. The effects of Althesin differ from those of other known anesthetics and suggest a unique mechanism. The possibility of action through naturally occurring steroid receptors is considered.