The effects of a chicken hypothalamic extract (HE) on the secretion of prolactin and growth hormone (GH) in vivo have been investigated by radioimmunoassay in the domestic fowl. Different i.v. doses of HE (0.25--25 HE equivalents/kg body weight) had no effect on GH secretion in conscious or anaesthetized cockerels. In both groups of birds the concentration of plasma prolactin was significantly increased within 10 min of administration of the extract. Extracts of other brain tissues (cerebral cortex, cerebellum and medulla oblongata) had no stimulatory effect on prolactin or GH secretion. Release of both prolactin and GH by dispersed pituitary cells and by hemipituitary glands in vitro was enhanced following incubation with HE (5 hypothalami equivalents/ml) or with single whole hypothalami respectively. Other brain tissues (cerebellum, optic lobes and medulla oblongata) had no effect on the concentration of prolactin or GH released by incubated hemipituitary glands.