In rodents, ubiquitous a1-Na+, K+-ATPase is inhibited by ouabain and other cardiotonic steroids (CTS) at ~103-fold higher concentrations than those effective in other mammals. To examine the specific roles of the CTS-sensitive a1S- and CTS-resistant a1R-Na+, K+-ATPase isoforms, we compared the effects of ouabain on intracellular Na+ and K+ content, cell survival, and mitogen-activated protein kinases (MAPK) in human and rat vascular smooth muscle cells (HASMC and RASMC), human and rat endothelial cells (HUVEC and RAEC), and human and rat brain astrocytes. 6-h exposure of HASMC and HUVEC to 3 µM ouabain dramatically increased the intracellular [Na+]/[K+] ratio to the same extend as in RASMC and RAEC treated with 3000 µM ouabain. In 24, 3 µM ouabain triggered the death of all types of human cells used in this study. Unlike human cells, we did not detect any effect of 3000-5000 µM ouabain on the survival of rat cells, or smooth muscle cells from mouse aorta (MASMC). Unlike in the wild-type a1R/R mouse, ouabain triggered death of MASMC from a1S/S mouse expressing human a1-Na+, K+-ATPase. Furthermore, transfection of HUVEC with rat a1R-Na+, K+-ATPase protected them from the ouabain-induced death. In HUVEC, ouabain led to phosphorylation of p38 MAPK, whereas in RAEC it stimulated phosphorylation of ERK1/2. Overall, our results, demonstrate that the drastic differences in cytotoxic action of ouabain on human and rodent cells are caused by unique features of a1S/a1R-Na+, K+-ATPase, rather than by any downstream CTS-sensitive/resistant components of the cell death machinery.