Significant decreases in blood pressure (BP) may occur when administration of angiotensin-converting enzyme (ACE) inhibitors is initiated for the treatment of heart failure. The purpose of this study was to compare the safety and tolerability of recommended initial doses of the longer-acting ACE inhibitor enalapril (ENAL) with those of the shorter-acting captopril (CAP) in patients with heart failure who were treated concomitantly with digitalis and diuretic agents. We evaluated BP, serum ACE activity, and clinical status when a low, first dose of ENAL (2.5 mg, n = 59) or CAP (6.25 mg, n = 58) was administered in a double-blind, randomized, and parallel fashion to 117 patients with mild to moderate heart failure. BP and serum ACE activity were measured at 30 min and hourly for 8 hours after drug administration. BP decreases were similar for both groups (mean supine BP -6.2/-4.8 mm Hg for ENAL vs -8.3/-6.4 mm Hg for CAP; mean standing BP -9.2/-5.6 mm Hg for ENAL vs -10.0/-4.7 mm Hg for CAP). Although the maximum mean decrease in BP occurred at hours 4 and 5 in the ENAL group and hours 1 and 2 in the CAP group, considerable between-group overlap was observed for individual patients. Decreases in mean serum ACE activity occurred earlier and were of shorter duration in the CAP group. ENAL significantly inhibited serum ACE activity to a greater extent than did CAP at all time points except the 1st hour. Administration of a first dose of ENAL, 2.5 mg or CAP, 6.25 mg to patients with heart failure was well tolerated.