By observing the histological changes in the brains after administering ACTH to monkeys and mice with experimental Japanese encephalitis, the author studied the effects of ACTH on pathological changes of the brain, and arrived at the following conclusions. 1. In the case of the mice, it has been found that ACTH tends to diminish inflammatory changes, and it markedly lessens the diffuse proliferation of glia cells. 2. The inhibitory effect on the diffuse proliferation of glia cells shows no parallel relationship with the dosage of ACTH administered. 3. In the case of the monkey with the ACTH administration, there can be observed a similar tendency to lessen the imflammatory changes and in addition, the demolition of nerve cells is inhibited. 4. With these findings as the basis, some of the therapeutic effects of ACTH on Japanse encephalitis have been discussed.