Incubation of human washed platelets with bovine aortic endothelial cells (ECs) treated with indomethacin resulted in an inhibition of thrombin-induced platelet aggregation that was dependent on the number of ECs added. Preincubation of ECs with Escherichia coli Iipopolysaccharide (LPS; 0.5-2.0 ,ug/ml) for 1 min signcantly enhanced their inhibitory activity. This effect was potentiated by superoxide dismutase (60 units/ml) and reversed by oxyhemoglobin (5-10 ,uM), indicating that the inhibition was due to the release of endothelium-derived relaxing factor (nitric oxide). When the ECs were pretreated with NG-monomethyl-L-arginine (30-300 ,IM) before LPS, the antiaggregatory activity was strongly reduced. The reduction of activity by NG-monomethyl-L-arginine was reversed by L-arginine (100 ,uM) but not by D-arginine (100 ,uM). Under similar conditions, LPS also enhanced the antiaggregatory activity of ECs grown on beads. The immediate enhancement by LPS of the release of endothelium-derived relaxing factor from endothelial cells may contribute to the rapid fall in blood pressure associated with endotoxin shock in vivo.Mammals infected with Gram-negative bacteria develop shock, characterized by hypotension, vascular injury, disseminated intravascular coagulation, and organ dysfunction (1). Escherichia coli is the most frequent causative agent in septic shock (2), the symptoms of which are primarily due to lipopolysaccharide (LPS). After a 4-hr incubation, LPS stimulates the generation of prostacyclin (PGI2) from endothelial cells (3-5), which may contribute to the vasodilation (through a cAMP-mediated relaxation of vascular smooth muscle) and decreased arterial blood pressure in shock (6, 7).Endothelium-derived relaxing factor (EDRF), which is released from the vascular endothelium in response to a number of stimuli (most of which also release PGI2), has been identified as nitric oxide (NO) (8,9). NO is a potent vasodilator (10) and inhibits platelet aggregation (11)(12)(13)