Dual chamber, rate-modulated pacemakers provide the capability of augmenting the heart rate of patients with chronotropic incompetence but also may cause atrial arrhythmias because of high rate, competitive atrial pacing. We studied ten patients with two consecutive 24-hour Holter monitors during which they were alternately programmed to either DDD or DDDR pacing in random order. Maximum heart rates (max HR) were measured at every 15-minute interval during each 24-hour period. DDDR pacing showed rate augmentation, 80 +/- 7 average max HR when compared with DDD pacing, average max HR 76 +/- 5. These results were even more striking when waking hours (7 am to 10 pm) were compared: average max HR 86 +/- 7 DDDR versus 78 +/- 4 average max HR DDD. Several patients showed marked rate augmentation. Seven of ten patients preferred DDDR pacing over DDD pacing. In the entire population, DDDR pacing did not result in an increased number of atrial arrhythmias (1.25 atrial events/24 hour) when compared to DDD pacing (1.75 atrial events/24 hour). We conclude that DDDR pacing provides heart rate augmentation during daily life in a clinical population while not resulting in a significant increase in atrial arrhythmias.