Newborn rats aged 3 days, removed from their mothers for 3 h, were given 0.4 ml of a 20% aqueous solution of human albumin by mouth. Pieces of tissue from the proximal jejunum and the renal cortex of the animals were investigated electron-microscopically by Gomori's reaction for acid phosphatase. In early postnatal ontogeny protein is absorbed in the intestine by pinocytosis, after which it is transported into the Golgi complex and discharged into the intercellular space, from which it enters the lymphatic capillaries. The accumulation of electron-dense protein-containing cytosomes, with high acid phosphatase activity, could be seen in the proximal tubules of the kidneys. It is postulated that the kidneys have a proteolytic digestive function in early postnatal ontogeny.