Hepatic glutathione S-transferase activities were determined with the substrates 1,2-dichloro-4-nitrobenzene and 1-chloro-2,4-dinitrobenzene. Sexual differentiation of glutathione S-transferase activities is not evident during the prepubertal period, but glutathione conjugation with 1,2-dichloro-4-nitrobenzene is 2-3-fold greater in adult males than in females. Glutathione conjugation with 1-chloro-2,4-dinitrobenzene is slightly higher in adult males than adult females. No change in activity was observed after postpubertal gonadectomy of males or females. Neonatal castration of males results in a significant decrease in glutathione conjugation with 1,2-dichloro-4-nitrobenzene. Hypophysectomy, or hypophysectomy followed by gonadectomy did result in significantly higher glutathione S-transferase activities in both sexes. These increases can be reversed by implanting an adult male or female pituitary or four prepubertal pituitaries under the kidney capsule. Postpubertal sexual differentiation of glutathione S-transferase activities is neither dependent on pituitary sexual differentiation nor pituitary maturation. Prolactin concentrations are inversely related to glutathione S-transferase activities in hypophysectomized rats with or without ectopic pituitaries. Somatotropin exogenously administered to hypophysectomized rats results in decreased glutathione S-transferase activities, whereas prolactin has no effect. Adult male rats treated neonatally with monosodium l-glutamate to induce arcuate nucleus lesions of the hypothalamus have decreased glutathione S-transferase activities towards 1,2-dichloro-4-nitrobenzene and decreased somatotropin concentrations. Our experiments suggests that sexual differentiation of hepatic glutathione S-transferase is a result of a hypothalamic inhibiting factor in the male (absent in the female). This postpubertally expressed inhibiting factor acts on the pituitary to prevent secretion of a pituitary inhibiting factor (autonomously secreted by the female), resulting in higher glutathione S-transferase activities in the adult male than the adult female.