The adhesion of human platelets to monolayers of bovine endothelial cells in culture was studied to determine the role of endothelium-derived nitric oxide in the regulation of platelet adhesion. The adhesion of unstimulated and thrombin-stimulated platelets, washed and labelled with indium-111, was lower in the presence than in the absence of bradykinin or exogenous nitric oxide. The inhibitory action of both bradykinin and nitric oxide was abolished by haemoglobin, but not by aspirin, and was potentiated by superoxide dismutase to a similar degree. It is suggested that the effect of bradykinin is mediated by the release of nitric oxide from the endothelial cells, and that nitric oxide release contributes to the non-adhesive properties of vascular endothelium.