The effects of the administration into the brain ventricle of histamine, selective H<sub>1</sub> and H<sub>2</sub>-receptor agonists and antagonists and chemically similar substances with nonspecific activity on basal and morphine-stimulated growth homone (GH) secretion in normal male rats were studied. None of the drugs had any significant effect on baseline rat GH levels, but histamine and H<sub>1</sub> agonists were able to decrease the rat GH release evoked by morphine. Mepyramine (H<sub>1</sub> antagonist) had no consistent effect by itself but was effective in preventing the inhibitory action of 2-methylhistamine (H<sub>1</sub> agonist). H<sub>2</sub> agonists and antagonists and their chemical analogues were all inhibitory, but by a mechanism which is nonspecific and must be interpreted cautiously. These results confirm the inhibitory effect of histamine on rat GH release and indicate that H<sub>1</sub> receptors in the CNS are responsible for this effect.