The postnatal developmental changes in the localization of nitric oxide synthase were studied in the spinal cord of albino rats using nicotina-mide adenine dinucleotide phosphate-diaphorase {NADPH-d) histochemis-try. At birth, NADPH-d positive neurons were present in the pericentral zone, the intermediolateral cell column and the deep dorsal horn (laminae IV-V). Moderately positive neurons were scattered in the ventral horn. NADPH-d positive nerve fibers were seen crossing to the opposite side in the pericentral zone, extending between the pericentral zone and tho-racolumbar intermediolateral cells and between cells in the deep laminae of the dorsal horn. In the white matter, fine nerve fibers were dispersed in all funiculi. In the thoracolumber region, neuronal processes of the sympathetic intermediolateral cells extended lat-
erally into the lateral funiculus.
At one postnatal week, more dense NADPH-d positive nerve fibers crossed the midline, dorsal and ventral to the central canal and in the ventral white commissure. Few cells (intercalated nucleus) were found medial to the intermediolateral cell column in the thoracolumbar region. The loosely arranged parasympathetic neurons were observed in the lateral part of lamina VII in the upper sacral segments. Small positive cells and nerve fibers appeared in lamina II of the dorsal horn. More positive cells appeared in the deep dorsal horn (laminae 111, VI), NADPH-d positive nerve fibers were more condense in the deep part of the dorsal funiculus.