Weanling pigs, dogs and rats were fed diets of different protein value. Some dogs and rats remained on the diets through pregnancy, and their offspring were also fed on the experimental diets. Measurements made during the life of the animals included intravenous glucose tolerance, insulin sensitivity, concentrations of growth hormone and insulin in plasma, as well as urinary corticosteroid excretion. The hypophyses, pancreata, thyroids and adrenals were examined histologically. Congenitally malnourished animals, and animals severely malnourished from weaning, showed marked histological changes, especially in the endocrine portion of the pancreas and in the thyroid. However, even a mild deficiency of protein disturbed the endocrine balance and produced marked aberrations in the control of carbohydrate metabolism, which persisted after apparent rehabilitation on a good diet. The experimental findings are discussed in relation to human deficiency syndromes such as marasmus and kwashiorkor.