2,5-Dibutyl-2,4-dihydro-4-[[2-(1H-tetrazol-5-yl)[1,1'-biphenyl]-4' - yl]methyl]-3H-1,2,4-triazol-3-one, SC-51316, was synthesized as a potent and orally active angiotensin II (AII) receptor antagonist with a long duration of action. To explore the lipophilic pocket in the AII receptor interacting with the substituent at the 2-position of triazolone-based antagonists, a series of compounds were prepared and evaluated for receptor binding affinity and antagonism of AII-contracted rabbit aortic rings. It has been found that the pocket is very spacious and can accommodate different sizes of lipophilic groups and various functionalities. Acidic groups generally result in a slight decrease in binding affinity. Branched chains are unfavorable. The freedom of rotation around C2-C3 in the flexible side chain is crucial for good binding. The 2-phenylethyl-substituted triazolone analogue exhibits the highest in vitro potency among all compounds that have been synthesized.