Nitric oxide ((NO)-N-.) plays an important role in a number of physiologic processes. Evidence exists that (NO)-N-., which stimulates soluble guanylate cyclase and enhances cyclic guanosine monophosphate (cGMP) levels, may inhibit platelet activation. In contrast, during platelet activation induced by different agonists, synthesis of (NO)-N-. in platelets occurs. In these studies, production of the stable end-products of (NO)-N-.-nitrite and nitrate (NOx) in human platelets, stimulated by different doses of lipopolysaccharide from Proteus mirabilis (LPS; endotoxin), has been evaluated. LPS is a weak platelet agonist that may activate various steps of platelet activation with the generation of reactive oxygen species. The mechanism of platelet activation induced by the endotoxin is not known. The aim of the present study was to measure the level of nitrite and NOx in blood platelets treated with LPS and to examine the level of nitrotyrosine in platelet proteins caused by LPS. Our results show that LPS at a low concentration (6.8 ng/ml) caused a decrease (approximately 80%) in the NOx level, whereas at higher concentrations (13.6 and 25 ng/ml) it induced an increase in the NOx level (approximately 210% and 260%, respectively). Our results indicate that LPS, like other agonists (thrombin, platelet-activating factor), can stimulate (NO)-N-. production in platelets. After incubating platelets with LPS, we also observed a distinct increase in platelet protein nitration (3-nitrotyrosine).