The metabolic response to human growth hormone (HGH) was studied in five obese subjects in the fed state and during prolonged (5-6 wk) starvation. In the fed state (three subjects), HGH induced an elevation in basal serum insulin concentration, a minimal increase in blood and urine ketone levels, and a marked reduction in urinary nitrogen and potassium excretion resulting in positive nitrogen and potassium balance. In prolonged fasting (four subjects), HGH administration resulted in a 2- to 3-fold increase in serum insulin which preceded a 50% elevation in blood glucose. Persistence of the lipolytic effects of HGH was indicated by a rise in free fatty acids and glycerol. The response differed markedly from the fed state in that blood beta-hydroxybutyrate and acetoacetate levels rose by 20-40%, resulting in total blood ketone acid concentrations of 10-12 mmoles/liter, ketonuria of 150-320 mmoles/day, and increased urinary potassium loss. The subjects complained of nausea, vomiting, weakness, and myalgias. Despite a 50% reduction in urea excretion during HGH administration, total nitrogen loss remained unchanged as urinary ammonia excretion rose by 50% and correlated directly with the degree of ketonuria. It is concluded that in prolonged starvation (a) HGH may have a direct insulinotropic effect on the beta cell independent of alterations in blood glucose concentration, (b) persistence of the lipolytic action of HGH results in severe exaggeration of starvation ketosis and interferes with its anticatabolic action by necessitating increased urinary ammonia loss, and (c) failure of HGH to reduce net protein catabolism in starvation suggests that this hormone does not have a prime regulatory role in conserving body protein stores during prolonged fasting.