We examined the mechanism of nitric oxide (NO) and/or superoxide (O2-)-induced cytotoxicity and the importance of thiols in endothelial cells by treating the cells with superoxide dismutase (SOD), catalase (CAT) and hemoglobin (Hb). Pyrogallol, a O2 generator and precursor of hydrogen peroxide (H2O2), had potent cytotoxic effects on the endothelial cells, but this effect was completely abolished by SOD/CAT. Hb, a NO scavenger, protected the endothelial cells from sodium nitroprusside-induced cytotoxicity. The cytotoxic effect of 3-morpholinosydnonimine (SIN-1), which is thought to form peroxynitrite (ONOO-) as a simultaneous O2- and NO generator, was completely blocked by SOD/CAT or Hb. On the other hand, pretreatment of endothelial cells with diethylmaleate, a glutathione depleter, aggravated the cytotoxicity induced by SIN-1, which was prevented by addition of exogenous glutathione and/or SOD/CAT. These data suggest that the cytotoxicity induced by NO, O2- and ONOO- can be blocked by glutathione, and that this is an important cellular protective mechanism against these reactive oxygen species.