The peripheral nerves of alloxan and streptozotocin diabetes rats six months after the induciton of diabetes were morphologically investigated. The effect of insulin treatment was also examined. Prominent segmental demyelination and remyelination were observed in both alloxan and streptozotocin diabetes rats by isolated nerve fiber studies. Axonal degeneration and globular swelling were also recognized in some nerve fibers. Transmission electron microscopic findings were characterized by the figures of destructed myelin sheaths and axons. Reduplication and thickening of basal lamina of vasa nervorum were noted in the diabetes rats. Scanning electron microscopy revealed three dimensional architectures of degenerated nerve fibers and rough surface of Schwann cells in the diabetes rats. Insulin treated diabetes rats showed less structural changes of nerve fibers. It was indicated that the peripheral nerve lesions of experimental diabetes rats were caused by metabolic impairment of both axons and Schwann cells. Insulin treatment seemed to be effective on the experimental diabetic neuropathy.