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Previous studies have demonstrated that losartan can block the thromboxane A2 receptor on the vascular wall. The aim of the present study was to assess the effect of losartan on human platelet activation.Platelets were obtained from 15 healthy men, aged 26–40 years. Platelet activation was measured by changes in the light transmission of platelet-rich plasma stimulated by the thromboxane A2 analog U46619 (5 × 10−6 mol/l) or ADP (10−5 mol/l).U46619-stimulated platelet aggregation was significantly inhibited by losartan in a dose-dependent manner. Only a high dose of EXP 3174 (5 × 10−5 mol/l), the in vivo active metabolite of losartan, was able to attenuate U46619-induced platelet activation. Captopril, an angiotensin I converting inhibitor, failed to modify U46619-induced platelet aggregation. Furthermore, the binding of [3H]-U46619 to platelets was competitively inhibited by losartan, whereas only a high dose of EXP 3174 reduced the binding of [3H]-U46619. Captopril failed to modify the binding of [3H]-U46619 to platelets. Losartan also reduced the platelet activation induced by ADP (10−5 mol/l), a platelet agonist partially dependent on thromboxane A2. In addition, when thromboxane A2 generation was blocked by aspirin, ADP-induced platelet aggregation was inhibited to a similar degree to the inhibition induced by losartan.Exogenous angiotensin II did not elicit any modification of either U46619- or ADP-stimulated platelet aggregation.Losartan decreased platelet aggregation by a thromboxane A2-dependent mechanism. EXP 3174 was less potent than losartan in reducing thromboxane A2-dependent platelet activation. Captopril and exogenous angiotensin II had no effect on human platelet activation. These results suggest that losartan reduced thromboxane A2-dependent platelet activation independently of its effect on angiotensin II.