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The aim of this study was to assess heart rate variability, a noninvasive parameter for studying the neurohumoral control of heart rate, in children with atrial septal defect. We examined five time-domain and three frequency-domain indices of heart rate variability determined from 24-hour Holter recordings in 20 children, aged 3-14 years, with secundum atrial septal defect who were about to undergo operation. The measures were compared to normal ranges and to the hemodynamic data of the preoperative cardiac catheterization. Most indices of heart rate variability were moderately depressed in children with atrial septal defect. Our data revealed negative correlations between indices and the mean right atrial pressure and the enddiastolic right ventricular pressure, respectively. The study demonstrates the importance of the right ventricular filling pressures for the autonomic control of the heart in children with atrial septal defect.