18 patients with grave DP treated with a long-term artificial pulmonary ventilation and feeding through a naso-gastric probe were studied. Biopsies of n. suralis taken at different periods after the appearance of the first signs of DP (from the 19th to the 69th day) were studied at light and electronic microscopy. The basis of DP is toxic myelopathy with paranodal demyelination mainly in the large myelinated neural fibers and a segmentary one in the smaller neural fibers. Axonal degeneration was observed in the gravest cases of DP and was secondary being the result of axon squeezing by a folded myelin and voluminous Schwann cell cytoplasm invaginated into the axon. In no case of DP inflammatory changes and(or) involvement of the immunocompetent cells were found. There was pronounced proliferation and activation of Schwann cells due to intensive utilization of the degradation products of myelin and remyelinization. Morphological signs of remyelinization were observed on the 35th day of DP in the presence of enhancing neurological symptoms. But remyelinization was not complete even on the 69th day of DP.