To assess the mechanism for the increased sinus rate after radiofrequency catheter ablation performed for atrioventricular nodal reentrant tachycardia (AVNRT), we studied heart rate variability before and after radiofrequency catheter ablation in 17 patients with AVNRT and in 38 patients with an accessory pathway. The accessory pathway was located at the left ventricular free wall, the right ventricular free wall, or the posterior interventricular septum. An increased sinus rate was observed in patients with AVNRT or with the accessory pathway at the posterior septum or left free wall after radiofrequency ablation. In these groups, high-frequency power, root mean square of successive difference and percent of adjacent cycles that were more than 50 ms apart, all of which are indices reflecting parasympathetic nervous activity, were decreased. The ratio of low-frequency to high-frequency power reflecting sympathovagal balance, was increased in patients with AVNRT or with an accessory pathway at the posterior septum or left free wall. Increases in sinus rate were correlated with decreases in high-frequency power, and percent of adjacent cycles more than 50 ms apart that the increase in heart rate was due to parasympathetic nervous withdrawal.