The goal of this investigation was to examine the interactions that can occur between platelet and endothelial cells, especially in human coronary arteries. In a first series of experiments, the effect of human endothelial cells (cultured on microcarrier beads) was tested on the aggregation of washed human platelets. Endothelial cells completely inhibited platelet aggregation. The prostacyclin-mediated anti-aggregatory component could be blocked with indomethacin. The remaining anti-aggregatory effect of endothelial cells could be prevented with the EDRF inhibitors, gossypol, haemoglobin and methylene blue. This indicates that the endothelial autoacoids, EDRF and prostacyclin, are both inhibitors of platelet aggregation. In a second approach we investigated the effect of aggregating platelets on the tone of human coronary arteries obtained during heart transplantation. Aggregating platelets produced strictly endothelium-dependent relaxations of coronary arterial strips. These relaxations were mainly mediated by EDRF, because inhibition of vascular prostacyclin synthesis with aspirin did not inhibit the relaxations significantly, but the relaxations were abolished by the aforementioned EDRF inhibitors. Platelet-derived adenosine 5'-diphosphate (ADP) is mainly responsible for the stimulation of EDRF production in coronary endothelial cells. ADP was an endothelium-dependent vasodilator, and apyrase (ADP-ase) completely prevented platelet-induced vasodilation. The other platelet products, serotonin and thromboxane A2, were weak and potent constrictors of coronary arteries, respectively. These data suggest that the ADP released by aggregating platelets stimulates the coronary endothelium to produce EDRF. EDRF overrides the constrictor effects of serotonin and thromboxane A2 and exerts an inhibitory effect against further platelet aggregation or adhesion.