Nitrogen equilibrium cannot be achieved in surgical patients without adequate nitrogen and calorie intake, frequently requiring central venous feedings. To investigate the hormonal environment under which nitrogen retention might be achieved during hypocaloric feedings, we administered peripheral vein nutrition with growth hormone (GH) to four healthy male patients. The intravenous diets provided 50% of estimated energy requirements (mean 566 kcal/m2/day) and adequate nitrogen (6 gm/m2/day). Each subject was studied for two 7-day periods; 10 mg of GH was given daily during one period and placebo during the other. Administration of GH decreased weight loss, caused retention of nitrogen, potassium, and phosphorus in amounts closely matching their proportions in skeletal muscle, and stimulated insulin production. GH also raised serum levels of free fatty acids and glycerol, increased urinary excretion of ketones, and favored fat oxidation in the postabsorptive state. Hyperinsulinemia and increased lipolysis and ketogenesis may augment the primary effects of GH. Further studies at adequate and approximately 30% of adequate calorie intake confirmed these findings. Maintenance of body protein with GH may allow improved nutritional care of catabolic patients that was previously unrecognized.