Substituted 2-sulfinylimidazoles were synthesized and investigated as potential inhibitors of gastric H+/K(+)-ATPase. The 4,5-unsubstituted imidazole series 6-11 and the 1,4,5,6-tetrahydrocyclopent[d]imidazole series 12 were found to be potent inhibitors of the acid secretory enzyme H+/K(+)-ATPase. Structure-activity relationships indicate that the substitution of 2-pyridyl groups at the 1-position of the imidazole moiety combined with (2-aminobenzyl)-sulfinyl groups at the 2-position leads to highly active compounds with a favorable chemical stability. Other substitution patterns in the imidazole moiety result in reducing biological activities. 2-[(2-Aminobenzyl) sulfinyl]-1-[2-(3-methylpyridyl)]-1,4,5,6-tetrahydrocyclopent++ ++ ++ [d]-imidazole (12h, T-776) was selected for further development as a potential clinical candidate. Extensive study on the acid degradation of 12h indicates a mechanism of action different from that of omeprazole, the first H+/K(+)-ATPase inhibitor introduced to the market.