Intravenous delivery of human umbilical cord blood (hUBC) cells in rat models of middle cerebral artery occlusion (MCAO) improves performance on tests of motor coordination when given 24 hrs after MCAO (Chen et al., Stroke 2001; Willing et al., J Neurosci Res 2003). Though this treatment time is sufficient for noticeable recovery there is in vitro evidence that delivery of hUCB between 48 and 72hrs may be more effective (Newman et al., SFN 2003). Astrocytic and microglial inflammatory responses to ischemia are at their peak at this time as are signals that attract intravenously administered hUCB cells to the site of injury. In the present study we set out to determine the optimal time at which delivery of hUCB was most beneficial for behavioral and neurological recovery. Sixty Sprague Dawley rats were transplanted 3hrs to 1mo post MCAO. MCAO-only rats served as a control. All rats were tested on a series of behavioral tests prior to stroke and again at 2 and 4wks post transplant. Similar to our previous ob...