A nonstressful frequent sampling technique was employed to determine the effect of perphenazine, 5 mg/kg on spontaneous secretion of rat growth hormone (rGH). The drug blocked secretion almost entirely for at least 3, but no more than 22 h. These results suggest a role for dopamine and/or norepinephrine as stimulatory neurotransmitters for spontaneous rGH secretory episodes. They further suggest that experimental or therapeutic use of phenothiazines might produce functional GH deficiency.