All experiments utilized blood samples obtained bychronic right atrial catheters. Growth hormone measurements, made by specific radioimmunoassay, showed that fasting and estrogen ad-ministration result in increased growth hormone secretion in the rabbit, although physical stress and exposure to cold do not. Insulin-induced hypoglycemia after fasting caused a decline in plasma growth hormone, and arginine infusion in the estrogen-primed rabbit resulted in an initial depression of plasma growth hormone followed by a renewed secretory phase. Electrical stimulation in the vicinity of the ventromedial nucleus of the hypothalamus caused an abrupt rise in serum growth hormone.