The angiotensin II receptor, losartan, has been found to inhibit platelet aggregability to some extent in in vitro experiments. There have been conflicting results about the in vivo effects of losartan. We sought to clarify the in vivo effect of losartan on platelet aggregation. Forty patients with grade I essential hypertension were treated with losartan for 3 weeks. Platelet aggregation tests with adenosine diphosphate (ADP) and ristocetin were analyzed and compared before and at the end of the study. Losartan effectively decreased systolic (SBP) and diastolic (DBP) blood pressure. Mean SBP before and after treatment was 159.6 +/- 12.8 and 149.2 +/- 17.3 mmHg, respectively. Mean DBP decreased from 93.7 +/- 8.2 to 87.7 +/- 10.3 mmHg after treatment. The results of the platelet aggregation tests with ADP and ristocetin were not significantly different when both rate and amplitude of maximal aggregation were included. Peak platelet aggregation with ADP regarding the lowest light transmission in the aggregometer was 59.8% +/- 24.3% before and 58.3% +/- 18.1% after the treatment. The same variables with ristocetin were 66.8% +/- 21.6% and 60.8% +/- 23.3%, respectively. In vivo effects of losartan on platelet aggregation with ADP and ristocetin were insignificant.