Fibrin and fibrinogen and their proteolytic degradation products, found within the atheroma, may contribute to vascular dysfunction. We monitored the production of endothelium-derived relaxing factor (EDRF) by bovine aortic endothelial cells (BAEC) after exposure to fibrinogen, fibrin monomer (FM), and fibrin and fibrinogen degradation products (FDP). Cells incubated with FM and FDP, compared with cells incubated with fibrinogen, were less able to inhibit platelet aggregation, and this effect correlated with a concentration-dependent decrease in EDRF production: BAEC incubated with FM or FDP, but not fibrinogen, produced significantly less nitric oxide (NO), as determined using a photolysis-chemiluminescence system. Northern analysis of BAEC incubated with fibrinogen, FM, or FDP and probed for constitutive bovine endothelial NO synthase mRNA demonstrated decreased expression in cells exposed to FDP or FM. These data show that FM and FDP reduce EDRF produced by BAEC and attenuate constitutive NO synthase expression. These effects may represent a mechanism by which thrombotic determinants adversely affect endothelial function and thereby potentially impair vasomotor responses and contribute to atherothrombogenesis.