Leucine treatment (100 mg/kg daily for 5 days) leads to activation of the hepatocyte nuclear system and granular endoplasmic reticulum and to a drastic increase in the number of mitochondria, characterized by polymorphism. In the spleen, Malpighian bodies and periarterial lymphoid sheaths are enlarged, lymphocytes infiltrate the periarterial zone, and the mantle zone is enlarged. In the thymus, the width of the cortical matter shrinks, while that of the medulla increases. The content of lymphocytes in the medulla decreases, while that of Hassal’s bodies increases. Unambiguous effects of leucine on the small intestinal morphology (mainly on the villous epithelium) were shown. Goblet cells in the villous epithelium were sharply stenosed because of decreased secretory granules in them.