The elevation of intracellular Ca2+ in various tissue through oxidative stress induced by menadione has been well documented. Increase of Ca2+ level in platelets results in aggregation of platelets. To test the hypothesis that menadione-induced Ca2+ elevations can play a role in platelet aggregation, we have studied the effect of menadione on aggregation of platelets isolated from female rats. Treatment with menadione to platelet-rich plasma (PRP), which proved to be an adequate system, appeared to induce dose-dependent platelet aggregations up to 60%, as determined by aggregometry. However, exposure of PRP to menadione led to slow reduction of platelet cell number coincident with a loss of viability, as measured by lactate dehydrogenase leakage, suggesting that menadione might induce cell lysis rather than aggregation of platelets. Light microscopy confirmed that menadione reduced the number of platelets and failed to show aggregates of platelets. To elucidate the mechanism of this cytotoxicity, menadione-induced oxygen consumption was studied in intact rat platelets. Incubation of platelets with menadione resulted in rapid dose-dependent increases of oxygen consumption, which were not inhibited by indomethacin and nordihydroguaiaretic acid, suggesting that menadione did not affect the cyclooxygenase and lipoxygenase pathways in platelets. Oxygen consumption, as well as cytotoxicity by menadione, was unaffected by addition of dicoumarol, which is a quinone reductase (QR) inhibitor. Consistent with these findings, no activity of QR was detected in any subcellular fractions of platelets. Oxygen consumption by several subcellular platelet fractions treated with menadione was examined in the presence of NADPH or NADH. Additions of NADPH or NADH to microsomal fractions or a 9000 g pellet (which contains plasma membranes) led to 2-fold to 18-fold elevations in platelets may contribute to the oxidative damage associated with menadione-induced oxygen consumption, respectively. These results suggest that NADPH and/or NADH-dependent enzyme systems in menadione-induced cytotoxicity.