Intact male and ovariectomized female rats were trained to discriminate 12 mg/kg pentobarbital from physiological saline. Generalization tests with different doses of pentobarbital did not reveal significant sex differences in the pentobarbital generalization gradient. Different doses of progesterone produced a generalization gradient to pentobarbital in ovariectomized females, but not in intact males. The results of the present experiment thus suggests that systemic administration of progesterone produces a "pentobarbital-like" stimulus in ovariectomized female rats, but not in intact males.