Intravenous injections of chlorpromazine(200 µg/100 g b.w.) or pentobarbital (2 mg/100 g b.w.) caused significant increases in both plasma growth hormone (GH) and prolactin levels in urethane-anesthetized rats. The concurrent administration of nicotine (0.02 mg/ 100 g b.w. i.p.) significantly blunted plasma GH responses to chlorpromazine and to pentobarbital, whereas plasma prolactin responses to these drugs were not affected. The inhibiting effect of nicotine on chlorpromazine-induced GH release was also observed in rats with hypothalamic deafferentation. These results suggest that nicotine suppresses GH secretion via the hypothalamus in rats.