The time course of c-jun expression and the effect of a peripheral nerve (PN) graft on axonal regeneration and c-jun expression in the lateral vestibular nucleus (LVN) were investigated in this study. c-Jun was found in LVN neurons 1 day after hemisection of the spinal cord at C3, with weak immunoreactivity. Strong c-Jun positive immunoreactivity was found 3 days post-injury. The expression of c-jun reached its maximum level (45% +/- 2.5, mean +/- SD) 1 week post-injury and gradually decreased to about a third of its peak value 6 weeks after the inflicted lesion. The PN graft implanted at the lesion site significantly enhanced the number of neurons expressing c-jun 4 weeks after PN transplantation (from 20% of LVN neurons in axotomy alone to 30% in PN graft transplantation, P < 0. 05). The PN graft induced injured neurons to regrow into the PN graft. About 91 FG-labeled neurons were found in the LVN and approximately 87% of these neurons showed c-Jun positive immunoreactivity. The high percentage of regenerating neurons expressing c-jun suggested that c-Jun might be involved in the regenerative process. The result of double staining for c-Jun and NOS showed that approximately 93% NOS positive neurons co-expressed c-Jun. Comparing the time course of the appearance of c-Jun with that of NOS, the expression of c-jun preceded that of NOS. Although a high percentage of NOS reactive neurons expressed c-jun, the expression of NOS in damaged LVN neurons needs to be further investigated.