The present study examines the effects of maternally administered halothane on fetal brain metabolism as determined by direct tissue analysis. Term pregnant rats were paralyzed, ventilated, and administered halothane in concentrations of 0.4, 1, or 2%. For comparison of fetal response to anesthetic agents, other maternal rats were administered pentobarbital (50 or 200 mg/kg). Dams receiving 0.4% halothane or 50 mg/kg pentobarbital remained normotensive, whereas 2% halothane or 200 mg/kg pentobarbital led to a 65% reduction in maternal blood pressure and a 3-fold increase in blood lactate. Fetal blood lactate tended to parallel the maternal lactacidemia. Fetuses of dams anesthetized with 0.4% halothane or 50 mg/kg pentobarbital exhibited concentrations of cerebral metabolities comparable to those of control animals. A 2% halothane level was associated with metabolic disturbances in fetal brain, indicative of cerebral hypoxia. Pentobarbital 200 mg/kg, although producing maternal hypotension and lactacidemia to a degree similar to 2% halothane, preserved a more optimal fetal cerebral energy state as reflected in a lower lactate/pyruvate ratio and normal ATP. The metabolic influence of pentobarbital may serve to protect the hypoxic fetus from neurological damage, an effect apparently not shared by maternally administered halothane.