OBJECTIVES: To compare the angiotensin II antagonistic properties of the usual recommended oral starting doses of various angiotensin II receptor antagonists-150 mg irbesartan, 80 mg valsartan, and 50 mg losartan-in humans. SUBJECTS AND METHODS: Eighteen healthy men were enrolled in a double-blind , randomized crossover study. Angiotensin II dose-effect curves of diastolic blood pressure and radioreceptor assay were performed before and up to 47 hours after single and multiple doses of the antagonists. The rightward shift of the angiotensin II dose-effect curves (dose ratio-1) assessed the antagonistic effects in vivo. The degree of receptor occupancy in plasma was detected by a rat lung radioreceptor assay ex vivo in vitro. RESULTS: All of the drugs clearly showed antagonistic effects to angiotensin II in vivo (dose ratio-1) and in vitro (radioreceptor assay). Within the given doses the dose ratio-1 for irbesartan was greater than for valsartan and losartan after single and repetitive dosing, reaching statistical significance at various time points up to 36 hours versus valsartan and up to 47 hours versus losartan. The apparent half-lives of the decay of the effects were approximately 8 hours for valsartan and losartan, whereas 15 to 18 hours were obtained with irbesartan. These findings were supported by the radioreceptor assay data: the percentage of receptor occupancy for irbesartan was significantly greater than for valsartan and losartan up to 47 hours. CONCLUSION: Angiotensin II antagonistic effects of irbesartan, valsartan, and losartan were compared . Irbesartan showed the slowest decay and longest duration of its antagonistic effects. With the recommended initial doses used in this study, the following rank order of antagonistic intensity was obtained : irbesartan > valsartan > losartan. The findings of this study, specifically the longer-lasting effects of irbesartan, may have clinical implications.