The neural cell adhesion molecule (N-CAM) is an integral membrane glycoprotein which mediates the adhesion of neurons to neurons and to other types of cells. During development, the N-CAM molecule is converted from a microheterogenous, highly sialylated, embryonic form (200-230 kDa) to several distinct, less sialylated but more adhesive, adult forms (120, 140 and 180 kDa). Monoclonal antibodies to epitopes of N-CAM (designated 5A5, 12F8, 5B8, 12F11 and 14E6) were used to investigate the spatial and temporal distribution of these neural cell adhesion molecules during the development of the corticospinal tract (CST) in rat spinal cord, from postnatal day 1 (P1) until adulthood. The light microscopical observations indicate that the embryonic form of N-CAM (200-230 kDa) recognized by 5A5 and 12F8 antibodies, respectively, is probably involved in the process of initial ingrowth of pioneer CST-fibers into the ventralmost part of the dorsal funiculus. The adult forms of N-CAM (120, 140, 180 kDa) recognized by 5B8, 12F11, and 14E6 antibodies, respectively, are present during later stages of CST white matter ingrowth and probably involved in fasciculation of the later arriving CST axons. During the period of CST target innervation (P4-P21), a gradual shift from embryonic (200-230 kDa) to adult (120, 140 and 180 kDa) forms of N-CAM occurs in spinal white matter and in the spinal gray matter adjacent to the ventral most part of the dorsal funiculus. The presence of embryonic N-CAM (200 kDa), with its low adhesive capacity in the CST outgrowth area may allow the CST axons to branch. If this branching is no longer desirable, only the higher affinity forms of N-CAM (120, 140 and 180 kDa) are expressed. The absence of N-CAM on CST target interneurons in the base of the dorsal horn and intermediate spinal gray matter strongly suggest that N-CAM is not involved in CST synapse formation. Together, these results suggest that various forms of N-CAM are involved in CST spinal white matter tract formation and subsequent target innervation, but not in the process of synapse formation.