EXP6155 (2-n-butyl-1-[4-carboxybenzyl]-4-chloroimidazole-5-acetic acid) and EXP6803 (methyl 2-n-butyl-1-[4-(2-carboxybenzamido)benzyl]-4-chloroimidazole -5-acetate, sodium salt) are shown to be novel, nonpeptide, antihypertensive, specific angiotensin II receptor antagonists. In rabbit aorta, they competitively inhibited the contractile response to angiotensin II with pA2 values of 6.54 and 7.20 and did not alter the response to norepinephrine or KCl. In guinea pig ileum, both agents blocked the responses to angiotensin I and II and did not alter the responses to bradykinin and acetylcholine. A similar specific angiotensin II antagonism was shown in vivo in the spinal pithed rat model. In renal artery-ligated rats, a high renin hypertensive model, EXP6155 and EXP6803 given intravenously, decreased blood pressure with ED30 of 10 and 11 mg/kg, respectively. Both compounds did not alter blood pressure when given orally at 100 mg/kg. Unlike saralasin, EXP6155 and EXP6803 given intravenously did not cause a transient increase in blood pressure in the renal artery-ligated and normotensive rats. Our results indicate that EXP6155 and EXP6803 are selective angiotensin II receptor antagonists and antihypertensive agents. Since neither compound had partial agonist activities or bradykinin potentiation effects, unlike the existing peptide angiotensin II receptor antagonists and angiotensin converting enzyme inhibitors, respectively, they may represent preferred probes for studying the physiological roles of angiotensin II.