For normal and above-normal concentrations of angiotensin-converting enzyme (ACE; EC 126.96.36.199) activity in plasma, results of a manual fluorometric method [with hippuryl-histidyl-leucine (HHL), 5 mmol/L, as substrate] correlated well with those of an automated spectrophotometric method [with 3-(2-furylacryloyl)-L-phenylalanyl-glycyl-glycine (FAPGG), 2 mmol/L, as substrate]. However, for patients receiving converting enzyme inhibitor (CEI) therapy, the spectrophotometric method showed much greater suppression of plasma ACE activity than did the fluorometric method. To determine which of the two methods provided a more reliable indication of ACE inhibition in vivo, we measured plasma ACE, angiotensin I (ANG I), and angiotensin II (ANG II) in patients receiving the CEI perindopril. During perindopril therapy, changes in the ratio of ANG II:ANG I, an index of ACE activity in vivo, showed a close agreement with changes in plasma ACE activity measured with FAPGG as substrate, but not with HHL as substrate. We conclude that measurement of ACE activity in vitro with FAPGG as substrate provides a reliable measure of changes in conversion of ANG I to ANG II in vivo during CEI therapy.