To measure effect of bone marrow stromal cells (MSCs) administered i.v. on rats subjected to traumatic brain injury (TBI), we injected MSCs labeled by BrdU into the tail vein 24 h after TBI and sacrificed rats 15 days later. The neurological severity score (NSS) and the Rotarod test were used to evaluate neurological function. The distribution of the donor cells in brain, heart, lung, kidney, liver and spleen were analyzed in recipient rats using immunohistochemical staining. MSCs injected i.v. significantly reduced motor and neurological deficits compared with control groups by day 15 after TBI. The cells preferentially entered and migrated into the parenchyma of the injured brain and expressed the neuronal marker NeuN and the astrocytic marker GFAP. MSCs were also found in other organs and primarily localized to the vascular structures, without any obvious adverse effects. Our data suggest that i.v. administration of MSCs may be useful in the treatment of TBI.