After administering ACTH to monkeys and mice with experimental Japanese encephalitis the author observed histological changes in the adrenal cortex, and obtained the following results. 1. In the case of the mouse adrenal cortex, Sudan stainable granules, Schiffsreaction; Schultzes reaction and double refractive substances tend to show a slight decrease and grow finer by the 48th hour after the inoculation of encephalitis virus, and these decrease markedly by the 72nd hour later, revealing congestion in all layers of the adrenal cortex. By the 96th hour there can be recognized the disappearance of Sudan stainable substances. 2. In the case of the mice with experimental encephalitis administered with 1 mg ACTH, Sudan stainable granules are markedly decreased already by the 48 th hour, leading them to death per se. 3. In the animals given 0.1 mg or 0.01 mg ACTH there can be recognized a decrease in the size and quantity of Sudan stainable granules by the 48th hour, but none of them show the disappearance of this substance, proving that ACTH at such a small dose delays the decrease in Sudan stainable substance. 4. In weighing the mouse adrenals of each group there can be recognized no uniformity in weight. 5. In a monkey not treated with ACTH and progressed acutely, hardly any histological changes can be observed in the adrenal cortex. In others which lifespan was over 6 days, treated with or without ACTH, there can be recognized no clear-cut difference in the adrenal cortices. 6. From these findings it is assumed that the adminstration of ACTH to the animals with experimental Japanse encepealitis does not increase the stress on the adrenal cortex as to debilitate the animal but it rather acts protectively on the function of the adrenal cortex. 7. As the administration of ACTH has an important bearing on the function of the adrenal cortext, it is advisable to pay a close attention to the fact that an excessive dose of ACTH will have a danger of bringing about undesirable effect.