Thrombin is a coagulation protease that activates platelets, endothelial cells, leukocytes and mesenchymal cells. Thrombin signaling is mediated at least in part by protease-activated receptors (PARs). As little is known about the in vivo regulation of PAR1, this study aimed to characterize the effects of systemic thrombin formation during human endotoxemia on the regulation of PAR1 and the associated responsiveness of human platelets to thrombin receptor activating peptide (TRAP). Endotoxin (2 ng/kg) was infused into 40 healthy men to study the regulation of PAR1 in systemic human inflammation. The SPAN12 antibody was used to determine the in vivo regulation of PAR1. To measure whether modulation of the PAR1 receptor may be associated with altered platelet reactivity, whole blood was stimulated with TRAP ex vivo. Thrombin generation was determined by prothrombin (F(1+2)) fragment. F(1+2) levels increased almost 9-fold from 0.5+/-0.1 nmol/L to 4.5+/-1.9 nmol/L at 4 h (p<0.001). PAR1 decreased by approximately 8% (p<0.001) within 2 h after endotoxin infusion and stayed at those levels until 6 h. Concomitantly, TRAP induced P-selectin expression maximally decreased by 18% (p<0.001) at 6 h. In conclusion, PAR1 expression is down-regulated on platelets during systemic thrombin formation induced by inflammation in humans which results in decreased responsiveness to subsequent stimulation of the PAR1 receptor.