We studied the inhibition of angiotensin converting enzyme (ACE) in eight infants with congestive heart failure (CHF) poorly controlled with digoxin and diuretics, treated orally with 0.25 mg kg-1 enalapril maleate once a day. Baseline ACE activities were compared between these infants and control children without CHF or ACE inhibitor. Except for one infant who vomited, inhibition of ACE activity was 75.5 +/- 12.2%, 75.5 +/- 10.5% and 51.7 +/- 12.2%, at 4, 12 and 24 h after drug intake respectively. There was no correlation between postnatal age and inhibition of ACE activity. In infants with CHF, mean baseline ACE activity was significantly higher than in control infants (36.4 +/- 7.2 mu ml-1 vs 26.9 +/- 6.9 mu ml-1, P < 0.05). These results were very similar to those seen in adults.