We demonstrated recently that transplantation of olfactory ensheathing cells from the nasal olfactory mucosa can promote axonal regeneration after complete transection of the spinal cord in adult rat. Ten weeks after transection and transplantation there was significant recovery of locomotor behaviour and restoration of descending inhibition of spinal cord reflexes, accompanied by growth of axons across the transection site, including serotonergic axons arising from the brainstem raphe nuclei. The present experiment was undertaken to determine whether olfactory ensheathing cells from the olfactory mucosa are capable of promoting regeneration when transplanted into the spinal cord 4 weeks after transection. Under general anaesthesia, thoracic spinal cord at the T10 level was transected completely in adult rats. Four weeks later, the scar tissue and cavities at the transection site were removed to create a 3-4 mm gap. Into this gap, between the cut surfaces of the spinal cord, pieces of olfactory lamina propria were placed. Ten weeks later, the locomotor activity of these animals was significantly improved compared with control animals, which received implants of either pieces of nasal respiratory lamina propria or collagen (Basso, Beattie, Bresnahan Locomotor Rating Scale scores 4.3 + 0.8, n = 6 versus 1.0 + 0.2, n = 10, respectively; P < 0.001). Ten weeks after transplantation the behavioural recovery was still improving. Regrowth of brainstem raphe axons across the transplant site was shown by the presence of serotonergic axons in the spinal cord caudal to the transection site, and by retrograde labelling of cells in the nucleus raphe magnus after injections of fluorogold into the caudal spinal cord. Neither serotonergic axons nor labelled brainstem cells were observed in the control animals. These results indicate that olfactory ensheathing cells from the nasal olfactory lamina propria have the ability to promote spinal cord regeneration when transplanted 4 weeks after complete transection. Olfactory ensheathing cells are accessible and available in the human nose; the present study further supports clinical use of these cells in repairing the human spinal cord via autologous transplantation.