Endothelial cell (EC) injury is observed in clinically important pathological processes, including bacterial endotoxemia. We hypothesized that such pathological processes may exhibit target organ heterogeneity due to organ-specific heterogeneity of endothelial cells. To test this hypothesis, endothelial cells of aorta (AO), pulmonary artery (PA), left ventricle (LV), and right ventricle (RV) were cultured from individual sheep and exposed to bacterial endotoxin. Marked heterogeneity in endotoxin-induced cytotoxicity was observed. AOEC were the most sensitive, followed by PAEC, LVEC, and RVEC. This cytotoxicity was manifested as programmed cell death (apoptosis). All cells were able to express both interleukin-6 and endothelin-1 (ET-1) transcripts. Following exposure to bacterial endotoxin, interleukin-6 transcripts accumulated in all cells, whereas ET-1 expression was constant or slightly decreased. These data suggest that organ-specific heterogeneity of EC responsiveness to endotoxin is a potential determinant of organspecific resistance to endotoxin and other mediators of injury.