It has been documented that beta-adrenergic antagonists can influence platelet aggregation by a mechanism independent of their ability to antagonize beta-adrenoceptors. Nebivolol, a selective beta1-adrenergic receptor antagonist with additional hemodynamic effects, is able to vasodilate human forearm vasculature by acting on the L-arginine/nitric oxide pathway. Constitutive nitric oxide synthase is present also in human platelets, resulting in the formation of nitric oxide, an endogenous inhibitor of platelet aggregation. The aim of this study was to investigate the effects of nebivolol on platelet aggregation and in particular to determine the involvement of the platelet L-arginine/nitric oxide pathway. Propranolol, a nonselective beta-adrenergic antagonist, and carvedilol, a beta-blocker with vasodilating properties, were compared with nebivolol on platelet activity. Plasma from healthy male subjects was used. Platelet aggregation was achieved with adenosine diphosphate (ADP) (3 microM) and collagen (1 microg/ml), using the Born turbidimetric method to measure platelet aggregation. Our results showed that nebivolol, propranolol, and carvedilol all had an inhibitory effect on both ADP- and collagen-induced platelet aggregation. Nebivolol exhibited the greatest inhibition effect on platelet aggregation. The mechanism responsible for the inhibitory effect of nebivolol appeared to involve a nitric oxide-dependent pathway. Indeed, L-arginine augmented the inhibitory effects of nebivolol on platelet aggregation induced by collagen and ADP. Furthermore, the inhibitory effect of nebivolol on platelet aggregation was reduced in the presence of the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA). In conclusion, we have demonstrated in this study that nebivolol's mechanism of platelet aggregation inhibition differs from that of other beta-adrenergic antagonists by being partially dependent on nitric oxide production.