Obese subjects show a subnormal growth hormone (GH) and prolactin (PRL) release in response to a variety of stimuli. Fenfluramine, an anorexiant drug used in obesity therapy, may have some effects on hypothalamic-pituitary function mediated by serotoninergic stimulation. The present investigation in obese subjects was carried out to study the effects of fenfluramine (60 mg orally) on GH and PRL secretion after intravenous arginine infusion. Ten volunteer obese females were studied and compared with 10 volunteer normal weight controls. In the obese group the GH response to arginine was significantly lower than in control group. Fenfluramine administration restored the subnormal GH response to arginine in obese subjects. The PRL response to arginine in obese women was subnormal. Fenfluramine administration restored the response of PRL to arginine infusion to normal. In conclusion, fenfluramine – under acute circumstances – enhances the hypothalamic-pituitary response to arginine in obese subjects. The decreased GH and PRL output in obese subjects is not due to an absolute hormonal deficiency and this effect of fenfluramine on GH secretion may – due to its lipolysis stimulation – be useful in obesity treatment.