Arginine aspartate was administered orally (250 mg/dg/day) for one week to 5 healthy male human volunteers aged 20-35. After this period, the 24 h patterns of growth hormone (GH) and prolactin (Prl) secretion were determined by radioimmunoassay on blood samples withdrawn every 20 min and correlated with the polygraphic recordings of electroencephalogram, electromyogram and electrooculogram. the results were compared to data obtained in identical conditions with the same subjects but after a week of placebo administration. In all 5 subjects the slow wave sleep related GH peak was about 60% higher after a week of arginine aspartate administration than in the control period, with individual changes of +24, +25, +42, +47 and +162%. The nocturnal mean plasma Prl of each subject was higher after arginine aspartate than before. The nocturnal rise of plasma Prl increased from a mean value of +21.5% during the placebo period to +95% at the end of the arginine aspartate treatment. These hormonal modifications were not accompanied by any detectable alteration of sleep organization and specially of the slow wave sleep pattern, suggesting a direct neuroendocrine action of the drug.