Single chamber rate responsive pacing offers many potential advantages over the more complex dual chamber atrial tracking pacing mode in children, and the preservation of atrioventricular synchrony could be unnecessary in selected groups of pediatric patients. Twenty-two pediatric patients (age range 9 months to 12 years; mean 6.5 years) had implantation of ventricular rate responsive (VVIR) pacemakers over a 2-year period. All patients had chronic third-degree atrioventricular block, and a normal ventricular function at rest. During the follow-up each patient underwent a 24-hour Holter monitoring, and ten performed a graded treadmill test in both ventricular fixed rate (VVI) and rate responsive (VVIR) pacing mode. Paced ventricular rates were found to be normal for age in all 22 patients; maximum rate did not reach the higher programmed rate during daily activities in any patient. Comparing the mean paced ventricular rate to the mean rates of blocked P waves, six patients showed a difference of more than 20 beats/min, which induced the pacemaker parameters to be reprogrammed. In all patients a significant correlation was found between variations of paced ventricular rate and variations of spontaneous blocked atrial rhythm (P < 0.05); this correlation persisted in the subsequent Holter controls in the ten patients with longer follow-up. Exercise tolerance resulted normal in the ten patients who performed a treadmill test either in VVIR or VVI mode, with increased maximal heart rates and maximal systolic blood pressure in VVIR mode (P < 0.0013). Rate responsive ventricular pacemakers seem to adequately respond to the physiological needs of daily life of this selected group of children requiring permanent pacing.