Heart rate variability (HRV) reflects the autonomic tone of the heart, and QT dispersion reflects the regional inhomogeneity of ventricular repolarization. The purpose of the present study was to determine the effects of early exercise training on HRV and QT dispersion in patients with acute myocardial infarction (AMI). Forty patients (mean age: 59 years) with AMI were randomized to training rehabilitation (group Tr, n=20) or conventional rehabilitation (group C, n=20). Two weeks after AMI, group Tr underwent 10 min of exercise using a bicycle ergometer (80% of anaerobic threshold) twice a day. At the end of the second and fourth weeks, 12-lead and 24-h Holter ECGs were recorded. QT intervals were measured and corrected using Bazett's formula (QTc), and QTc dispersion (QTcd) was defined as the difference between maximum and minimum QTc. HRV was accessed by the high-frequency component (HF: 0.15-0.40 Hz) of the HRV power spectrum (parasympathetic activity) and the ratio of low frequency (0.04-0.15 Hz) to HF (L/H ratio: sympathetic activity). In group Tr, HF increased (82.5 to 131.1 ms²), the L/H ratio decreased (3.9 to 2.6), and QTcd decreased (77.2 to 57.2 ms). In group C, none of the indices changed. It was concluded that early exercise training improves sympathovagal balance and decreases QTcd, and may reduce the arrhythmogenic substrate following AMI. (Jpn Circ J 1999; 63: 577 - 582)