To evaluate a possible role of histamine in the CNS control of prolactin and gonadotropin release, adult ovariectomized rats, with stainless steel cannulae implanted in the 3rd ventricle, were given s.c. injections of 10 mug of estradiol benzoate. 48 h later, 2 mul of 0.9% NaCl alone or of saline containing 1, 5, 25 or 125 mug of histamine dihydrochloride was microinjected into the ventricle. Immediately before and then 15, 30 and 60 min after, blood samples were withdrawn from etherized rats for radioimmunoassay (RIA) of serum prolactin, LH and FSH. In the histamine-injected rats, an increase in prolactin titers was observed and was highly significant in groups receiving the higher doses. A small yet significant release of LH, but not of FSH, was also observed. When 25 mug of histamine was injected directly into the pituitary or into the jugular vein, no elevations were observed, indicating a site of action in the brain. Restraint stress elevated serum prolactin and lowered serum LH and FSH in ovariectomized rats. These responses were blocked by the intrajugular injection of diphenhydramine (5 mg/kg). It is suggested that histamine may be involved in the hypothalamic control of prolactin release and possibly of gonadotropin release.