Abstract: An atrial pacemaker having an atrial pacing rate adjusted to correspond to a detected ventricular heart rate to maintain A-V synchrony in a range between a minimum and maximum pacing rate. Sensed ventricular depolarizations not associated with a preceding atrial pacing stimulus cause the atrial pacing rate to be increased. When A-V synchrony is restored at a higher rate, the rate is decreased. Underdrive stimulation to treat pathologic tachycardia is achieved as the atrial pacing rate is increased in steps to the maximum pacing rate, normally lower than typical tachycardia rates.