The vascular endothelium produces factors that modulate platelet-vessel wall interactions. Prostacyclin and endothelium-derived relaxing factor (EDRF), which is identified as nitric oxide (NO), are key inhibitors of platelet aggregation. This study examines the interactions between NO and prostacyclin in endothelial cells and their effects on platelet aggregation. When porcine aortic endothelial cells are stimulated with bradykinin (Bk), they release NO and prostacyclin. Indomethacin inhibits prostacyclin release but not NO. In the absence of indomethacin, low concentrations of Bk release small amounts of both NO and prostacyclin, which synergistically inhibit platelet aggregation. Both NO and prostacyclin inhibit platelet aggregation, and their anti-aggregating activities are reciprocally potentiated. NO and prostacyclin also cause platelet disaggregation. The anti-aggregating activity of NO and prostacyclin is enhanced by SOD and M&B 22948, a selective inhibitor of cyclic GMP phosphodiesterase. However, the presence of Hb, which inactivates NO, reduces this activity. The study shows that NO and prostacyclin are released by endothelial cells and synergize to inhibit platelet aggregation. These interactions may play a role in maintaining platelet homeostasis. The findings suggest that prostacyclin and NO may regulate platelet-vessel wall interactions in vivo at concentrations too low to be detected by non-biological methods. The study also indicates that impairments in the production or interaction of these substances may contribute to conditions such as thrombosis, vasospasm, and atherosclerosis.The vascular endothelium produces factors that modulate platelet-vessel wall interactions. Prostacyclin and endothelium-derived relaxing factor (EDRF), which is identified as nitric oxide (NO), are key inhibitors of platelet aggregation. This study examines the interactions between NO and prostacyclin in endothelial cells and their effects on platelet aggregation. When porcine aortic endothelial cells are stimulated with bradykinin (Bk), they release NO and prostacyclin. Indomethacin inhibits prostacyclin release but not NO. In the absence of indomethacin, low concentrations of Bk release small amounts of both NO and prostacyclin, which synergistically inhibit platelet aggregation. Both NO and prostacyclin inhibit platelet aggregation, and their anti-aggregating activities are reciprocally potentiated. NO and prostacyclin also cause platelet disaggregation. The anti-aggregating activity of NO and prostacyclin is enhanced by SOD and M&B 22948, a selective inhibitor of cyclic GMP phosphodiesterase. However, the presence of Hb, which inactivates NO, reduces this activity. The study shows that NO and prostacyclin are released by endothelial cells and synergize to inhibit platelet aggregation. These interactions may play a role in maintaining platelet homeostasis. The findings suggest that prostacyclin and NO may regulate platelet-vessel wall interactions in vivo at concentrations too low to be detected by non-biological methods. The study also indicates that impairments in the production or interaction of these substances may contribute to conditions such as thrombosis, vasospasm, and atherosclerosis.