Protamine zinc insulin has no growth-promoting effect in rats whose growth has been inhibited by diethylstilboestrol. Hypophyseal growth hormone stimulates growth in diethylstilboestrol-treated rats, and treatment with growth hormone and protamine zinc insulin results in an enhanced rate of growth. Protamine zinc insulin has growth-promoting properties in hypophysectomized rats, and cortisone acetate inhibits growth in insulin-treated hypophysectomized rats. 2-4-Dinitrophenol prevents growth and increases nitrogen excretion in young rats. Combined treatment with insulin, growth hormone and glucose induces growth and brings about positive nitrogen balance in dinitrophenol-poisoned rats. Adrenal, testis, liver and pancreas weights are normal in dinitrophenol-poisoned rats. The amount of extractable pancreatic insulin is also within normal limits. It is suggested that the functions of the hypophysis and pancreas are not seriously impaired, and that treatment with growth hormone and insulin promotes esterification of inorganic phosphate to high-energy phosphate compounds in the dinitrophenol-poisoned rats.