In vitro studies have shown that telmisartan is an insurmountable angiotensin II subtype-1 (AT1) receptor antagonist. Herein, the molecular basis of this insurmountable antagonism has been investigated in vitro, and the effect of telmisartan has been compared in vivo with that of irbesartan and candesartan. Association and dissociation kinetics of telmisartan to AT1 receptors have been characterized in vitro on rat vascular smooth muscle cells (RVSMC) expressing solely the AT1 receptor subtype. In a second set of experiments, the antagonistic efficacy of single intravenous doses (0.1, 0.3, and 1 mg/kg) of telmisartan was compared with that of irbesartan (0.3, 1.0, 3.0, and 10.0 mg/kg) and candesartan (0.3 and 1 mg/kg) in conscious, normotensive, male Wistar rats. The results show that the specific binding of [(3)H]telmisartan to the surface of living RVSMC is saturable and increases quickly to reach equilibrium within 1 h. Telmisartan dissociates very slowly from the receptor with a dissociation half-life (t(1/2)) of 75 min, which is comparable with candesartan and almost 5 times slower than angiotensin II (AngII). In vivo, telmisartan blunts the blood pressure response to exogenous AngII dose dependently. The blockade is long lasting and remains significant at 24 h at doses >0.1 mg/kg. Ex vivo assessment of the AT1 receptor blockade using an in vitro AngII receptor binding assay shows similar results. When administered intravenously in rats, telmisartan is 10-fold more potent than irbesartan and comparable to candesartan. Taken together, our in vitro data show that the insurmountable antagonism of telmisartan is due at least in part to its very slow dissociation from AT1 receptors.