SummaryRats receiving a fixed anesthetic dose of pentobarbital developed hypersensitivity to the drug 28 days following tolerance induction. This delayed increase in sleeping time was non-significant in the male, but significant in the female animal (P = 0.05). Tolerant female rats showed an increase in liver weight (as % body weight) and an enhanced rate of disappearance of barbiturate from blood. Hypersensitive female rats failed to exhibit a significant change either in liver weight on in rate of decrease of barbiturate blood level compared to naive animals. Delayed hypersensitivity to pentobarbital in the female rat appears, therefore, to be unrelated to alternations in hepatic metabolizing enzyme activity.