A novel series of heterocyclic compounds I bearing two acidic functionalities, a carboxyl group and a tetrazole ring, was prepared and evaluated for in vitro and in vivo angiotensin II (AII) antagonistic activity. These compounds showed significantly more potent AII antagonistic activities than the parent compounds without the carboxyl groups. This structure-activity relationship (SAR) study revealed the importance of the carboxyl group attached to the heterocyclic moieties especially for insurmountable antagonism and enhancement of in vivo (po) activity.