Between July 1990 and September 1991, 30 infants and children, most of whom had a congenital heart defect and who had been treated at least during the previous 20 days by two daily doses of digoxin and were in a stable clinical condition, were selected at random. A maintenance dose of digoxin was administered at 24-h intervals for 7 days in the study group (n = 15); no change was made in the 12-h dosage interval in the control group (n = 15). When the serum digoxin concentrations were compared, no significant difference was found between pre- and poststudy values in the study group (1.0 +/- 0.6 and 0.8 +/- 0.3 ng/ml, respectively) or between the control and study groups (0.9 +/- 0.6 and 0.8 +/- 0.3 ng/ml, respectively) in terms of trough serum digoxin concentrations. Although the peak serum concentrations in the study group were increased significantly (2.3 +/- 0.8 ng/ml) compared with prestudy peak levels (1.6 +/- 0.7 ng/ml, p < 0.05) and with the level in the control group (1.5 +/- 0.8 ng/ml, p < 0.05), a toxic concentration was not reached, and toxicity symptoms were not observed clinically. Blood pressure, heart rate, and liver size did not change significantly in any patient during the study.