The role of thrombin in inducing adherence of platelets to cultured vascular cells is complex and may involve factors of both platelet and vascular origin. Dansylarginine N- (3-ethyl-l,5-pentanediyl)amide, DAPA, a fast-acting inhibitor of thrombin, was used in this study to evaluate platelet activation and endothelial PGI2 release induced by thrombin. Monolayers of endothelial cells and fibroblasts were cultured from human umbilical cord vessels. Empty culture dishes were used as a control. Some vascular cells were treated with aspirin (ASA) to inhibit PGI2 formation before they were incubated with 51Cr-platelets for platelet adherence studies. PGI2 was assayed by radioimmunoassay for 6-keto-PGF1α. In one study, endothelium was incubated with 0.3 U thrombin for 2 min. before 10 μM DAPA was added. Platelets were next mixed with this solution and adherence was determined. DAPA caused a decrease in thrombin-induced platelet adherence to normal endothelium from 8% to 2%. When ASA- treated endothelium was employed, DAPA decreased adherence from 59% to 2%. In another study, 51cr-platelets were first aggregated with 0.1 U thrombin, then 1.3 μM DAPA was added, and these aggregates were incubated with the monolayers. Similar experiments without DAPA served as positive controls. DAPA did not reverse the aggregate formation, but it did reduce platelet adherence to normal endothelium from 38% to 1%, and to ASA-treated endothelium from 68% to 11%. In contrast, DAPA had little effect with fibroblasts (81% to 71%), ASA-treated fibroblasts (82% to 67%), and the empty dish control (86% to 66%). These results suggest that thrombin-induced platelet adherence in this system involves more than just an effect upon platelets. In addition, they provide further evidence that the non-thrombogenic nature of endothelium persists despite the absence of PGI2.