Male albino rats fed from weaning on a diet containing no protein lose body weight in spite of an adequate caloric intake per kilogram body weight. While loss of body weight is primarily associated with loss of weight in muscle and skin, it is particularly marked in certain organs such as liver, salivary glands, spleen, testes, and thymus gland. Loss of weight appeared to be due to inhibition of development and growth of body organs; for example, the red pulp of the spleen, the thymocytic lobules in the thymus gland, and spermatogenic tissue in the testes were undeveloped. Loss of weight may have been due also to impaired digestion of food since there were few zymogenic granules in the serous salivary glands and in the acinar glands of the pancreas. The terminal effect was the gradual development of an acidotic hypothermic cachexia, with death after 6–8 weeks from weaning, in the rats fed a diet containing no protein.