The effects of bone marrow stromal cells (BMSCs) on the repair of injured spinal cord and on the behavioral improvement were studied in the rat. The spinal cord was injured by contusion using a weight-drop at the level of T8-9, and the BMSCs from the bone marrow of the same strain were infused into the cerebrospinal fluid (CSF) through the 4th ventricle. BMSCs were conveyed through the CSF to the spinal cord, where most BMSCs attached to the spinal surface although a few invaded the lesion. The BBB score was higher, and the cavity volume was smaller in the rats with transplantation than in the control rats. Transplanted cells gradually decreased in number and disappeared from the spinal cord 3 weeks after injection. The medium supplemented by CSF (250 microl in 3 ml medium) harvested from the rats in which BMSCs had been injected 2 days previously promoted the neurosphere cells to adhere to the culture dish and to spread into the periphery. These results suggest that BMSCs can exert effects by producing some trophic factors into the CSF or by contacting with host spinal tissues on the reduction of cavities and on the improvement of behavioral function in the rat. Considering that BMSCs can be used for autologous transplantation, and that the CSF infusion of transplants imposes a minimal burden on patients, the results of the present study are important and promising for the clinical use of BMSCs in spinal cord injury treatment.