FOSINOPRIL SODIUM IS HYDROLYZED BY ESTERASES TO THE PHARMACOLOGICALLY ACTIVE FORM, FOSINOPRILAT, A SPECIFIC COMPETITIVE INHIBITOR OF ANGIOTENSIN CONVERTING ENZYME (ACE). ACE IS A PEPTIDYL DIPEPTIDASE THAT CATALYZES THE CONVERSION OF ANGIOTENSIN I TO THE VASOCONSTRICTOR SUBSTANCE, ANGIOTENSIN II. ANGIOTENSIN II ALSO STIMULATES ALDOSTERONE SECRETION BY THE ADRENAL CORTEX. INHIBITION OF ACE RESULTS IN DECREASED PLASMA ANGIOTENSIN II, WHICH LEADS TO DECREASED VASOPRESSOR ACTIVITY AND TO DECREASED ALDOSTERONE SECRETION.