Wistar strain male rats (body weight 180 g) were given 0.5 mg beta-methasone per kilogram of body weight orally each day for 180 days. Histopathologically, remarkable changes occurred only in the hypothalamus and thalamus. The mitochondria showed vacuolar degeneration not only in the nerve cells but also in the nerve fibers and glia cells, and the Golgi apparatus was well developed in the nerve cells. By the freeze fracture method, the intramembranous large globular molecules of mitochondria and nuclei of the nerve cells decreased in number. The steroid affects mainly the hypothalamus and thalamus. This might have a close relation to steroid psychosis.