Spirapril is a new angiotensin-converting enzyme (ACE) inhibitor with a long duration of action. To determine whether duration of inhibition of serum ACE activity may affect regional blood flow (RBF), we compared spirapril with captopril, an ACE inhibitor with a short duration of action. Both the short- and long-term effects were studied in patients with mild to moderate congestive heart failure (CHF). Calf, renal, and hepatic BF measurements were performed in the morning before intake of the study medication; 24 h after the previous dose of spirapril (n = 9 patients) and 12 h after the previous dose of captopril (n = 9 patients). Serum ACE activity after 1, 6, and 12 weeks was significantly reduced in patients receiving spirapril, but not in those receiving captopril. The decrease in mean arterial pressure (MAP) was more pronounced in the spirapril group. Calf BF showed a slight but not significant increase in both spirapril- and captopril-treated patients. Effective renal BF increased significantly only in patients treated with spirapril. Although filtration fraction (FF) tended to decrease in the spirapril group, the decrease was significant only in the captopril group. No changes were observed in hepatic BF. Cerebral BF (CBF) measurements were performed after intake of the first dose of study medication and after 12 weeks, immediately after drug intake. Significant reduction in MAP in the two treatment groups both after the first dose and after 12 weeks did not affect CBF. Despite a significantly prolonged decrease in MAP and serum ACE activity in spirapril-treated patients, no marked differences in RBF were noted between the two ACE inhibitors.