Animal experiments, analysis of growth hormone (GH) secretion in the human, studies on the synergistic metabolic effects between GH and sex hormones using the stable isotope 15N, and long-term growth evaluation of patients with GH and gonadotrophin deficiency treated with testosterone and hGH show that biologically important interrelations exist between GH and sex hormones. They are operative not only at the pituitary and gonadal levels but also in peripheral tissues, where sex hormones modify the metabolic and growth-promoting effects of GH. GH is also important for an optimal androgenic effect of testosterone in development of secondary sexual characteristics. Knowledge of these interrelations is not only of physiological interest, but also of practical clinical value.