Serum human growth hormone (HGH) content was determined in 150 participants by a two-antibody immunoassay. Some subjects concealed taking medication, especially progestogens causing increases of HGH. Physiologic doses of estrogens increased serum HGH values in fasting, ambulant women but not in men. High HGH values were found in patients with acromegaly or acromegaly-gigantism, in patients with anorexia nervosa or islet cell tumor of the pancreas, and in one with chromophobe adenoma of the pituitary. Exercise and insulin- or tolbutamide-induced hypoglycemia were the most reliable provocative tests for HGH. Intravenous arginine infusion was least reliable. Exaggerated HGH responses to hypoglycemia were observed in a patient with persistent lactation and amenorrhea; and impaired responses were observed in patients with anorexia nervosa, pituitary basophilism (Cushing's disease) thyrotoxicosis, and obese subjects.