Phase and electron microscopic study of sciatic and peroneal nerves of thiamine deficient and chronically starved rats demonstrated an increase in the incidence of normal variations in myelin sheath contour throughout the internode, focal alterations in the lamellar pattern of myelin adjacent to sites of demyelination, and numerous, complex, redundant loops of axon-Schwann cell membrane which projected into axons of myelinated fibers. The paranodal axoplasm of a few fibers from thiamine deficient rats contained collections of vesicles, mitochondria and dense bodies. The significance of these observations is briefly discussed.