Androgenized, oestrogenized and control female and male rats were used to establish possible differences in the alteration of the prolactin control system. Neonatal treatment involved administration of oestradiol benzoate or testosterone propionate (TP) on days 1 and 5 to the males and on day 5 to the females. Oil-treated animals were used as controls. Plasma prolactin levels were measured in these animals during adulthood (a) before gonadectomy, performed on day 80, and 27 days after gonadectomy and (b) on the 2 days (at 10.00 and 17.00 h) after administration of a single dose of TP to gonadectomized animals. Oestrogenized rats had the highest plasma prolactin concentrations just before and after gonadectomy. Testosterone propionate increased plasma prolactin levels in all groups. This response was more notable in the female than in the male groups, and was highest in the oestrogenized animals. Temporal rhythms of the prolactin response to TP were daily, perhaps circadian, with increased levels in the afternoon compared with those in the morning. This pattern was not present in oestrogenized females and androgenized males. Results suggest (a) that oestrogens and androgens given neonatally differ in their ability to alter the prolactin control system, and (b) that females seem to be more sensitive than males to changes in hypothalamic differentiation induced by neonatal steroid treatment.