We have compared the effects of intracellular pathways initiated by nitric oxide and prostacyclin on thrombin-induced platelet adhesion to endothelial cells. Platelet aggregate adhesion was enhanced when endothelial monolayers were pretreated with NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide production. In addition, decreased platelet aggregate adhesion was seen when platelets were pretreated with 8-bromoadenosine 3',5'-cyclic monophosphate (8-bromo-cAMP) or 8-bromoguanosine 3',5'-cyclic monophosphate (8-bromo-cGMP). Single platelet adhesion in isolated perfused lungs under flow conditions in the presence of shear was also assessed. Pretreatment of platelets with either Iloprost, in a dose sufficient to decrease platelet aggregation, or 8-bromo-cAMP did not affect platelet adhesion. However, pretreatment of platelets with 8-bromo-cGMP significantly reduced single platelet adhesion to endothelium. These studies illustrate that nitric oxide inhibits platelet adhesion to endothelium in the presence of shear. They further indicate that prostacyclin is also a regulator of this response but has effects more specifically related to the inhibition of platelet aggregation than platelet-endothelium interactions.