Distribution and intensity of NADPH-d reactivity, a marker for enzyme of the nitric oxide synthesis, in nervus suralis biopsies in severe DP were studied at light and electron microscopic levels. The study of control specimens has shown that NADPH-d reactivity was permanently present in Schwann cells (SC) and was distributed in all parts of their cytoplasm. Axon and myelin were devoid of NADPH-d reactivity. A decrease of enzyme reactivity in SC cytoplasm of the damaged nerve fibers and rising enzyme reactivity in the cytoplasm of activated SC were observed in DP. High reactivity in SC of small fibers was found at earlier stages and that of thick fibers at later stages. This distinction reflected, apparently, sequence of entering at first thin, then thick fibres in the reparative process. Under the electron microscope, the reaction product was deposited on membranes of endoplasmic reticulum, nuclear membrane and Golgi complex. The enzyme was also located in nucleus of activated SC. Ultrastructural location and the fact that the highest intensity of reaction is present in SC of nervous fibres with morphological signs of remyelination suggest link of this enzyme with the reparative process. This study provides the first evidence of NADPH-d reactivity in SC and shows that NADPH-d histochemistry is a useful tool for peripheral nerve biopsies study.