Our evidence indicates that brainstem endothelium is a viable target for controlling arterial pressure (Waki et al. 2007). Since endothelial progenitor cells (EPCs) have been shown to be possible vehicles for targeting the endothelium, we used EPCs to target the brainstem microvasculature in vivo and assessed any effect on cardiovascular function. Bone marrow mononuclear cells from male Wistar rats were isolated using a Histopaque gradient and expanded in conditions appropriate for the endothelial lineage (Caporali et al. 2008). Endothelial progenitor cells were characterized by morphology, immunofluorescence, and using the Matrigel assay for network formation (Hur et al. 2004). Rats were injected (under anaesthesia as below) with ~90,000 fluorescently tagged EPCs (CM-DiI, Invitrogen) on day 7 of culture into the nucleus tractus solitarii (NTS), a pivotal region regulating arterial pressure (AP). Rats anaesthetised (ketamine 60mg/kg and domitor 250µg/kg, i.p.) were implanted with a radio-telemetry device for AP measurement. Animals were allowed to recover for 7 days before 48hr baseline AP was recorded prior injection of either EPCs or Dulbecco’s Phosphate Buffered Saline (DPBS, without calcium and magnesium) vehicle. Recordings were made for up to 32 days following injection. Telemetry data was analysed using the Hey Presto software (Waki et al. 2006). Cells were characterised over 15 days cultured bone marrow cells showed the characteristic spindle and cobblestone morphology. On day 7 of culture, cells could take up acetylated low density lipoprotein-DiI and bind to BS-lectin-FITC and were able to form characteristic networks on Matrigel providing evidence for EPC identity. Following brainstem injection EPCs fluorescently tagged with CM-DiI migrated randomly away from the site of injection becoming closely associated with the vasculature. Between 10-15 days post EPC injection systolic pressure (SP), low frequency spectra of SP and heart rate were all reduced (respectively: -4.5±1.3 mmHg, P=0.009; -14.9±6.7%, P=0.04; -10.4±6.6 beats/min, P=0.075; paired students t-test, n=5). No changes in these variables were seen in the vehicle control group (n=5). In conclusion, EPCs injected into NTS remain viable for at least 32 days when transplanted into the brainstem in vivo, integrate into the vascular wall, and may alter cardiovascular autonomic function in normotensive rats.