The endothelium releases factor(s) that are potent vasodilators and inhibitors of platelet aggregation. Experiments were performed to determine whether the endothelium-dependent responses to aggregating platelets are altered in vein grafts. Segments of jugular veins were grafted in the reverse position into the carotid arteries in 16 rabbits. After 4 weeks the patent grafts (14 of 16) were removed, and the endothelium-dependent responses were examined in vitro. In control veins aggregating platelets, adenosine diphosphate, and serotonin caused endothelium-dependent relaxations. The platelet-induced relaxations were attenuated by apyrase (adenosine diphosphatase and adenosine triphosphatase) but not by methiothepin (serotonergic blocker). In vein grafts, endothelium-dependent relaxations in response to aggregating platelets were absent, and only contractions that could be attenuated by methiothepin were observed. In vein grafts, endothelium-dependent relaxations in response to adenosine diphosphate were reduced, and only endothelium-independent contractions were observed in response to serotonin. These contractions were attenuated by methiothepin. These results suggest that (1) the endothelium exerts an inhibitory effects mediated mainly by adenosine diphosphate in response to aggregating platelets in rabbit jugular veins and (2) endothelium-dependent relaxations in response to aggregating platelets are impaired in vein grafts because of reduced endothelium-independent contractions in response to serotonin. This impairment of endothelium-dependent responses in vein grafts may contribute to failure of the grafts.