Abstract: A pacemaker has circuitry which allows the time between the detection of an atrial contraction and the provision of an electrical stimulus to cause a ventricular contraction to vary with the rate of the sensed atrial contractions. The circuitry includes a differential amplifier having a reference voltage applied to one input and a capacitor coupled to the second input. The capacitor is charged from the time the atrial contraction is sensed to the time the charge equals the reference voltage, at which time the electrical stimulus is provided and the capacitor is discharged between the time the electrical stimulus is provided and the next atrial contraction is sensed. As the heart rate increases, the time between the atrial contractions decreases, and thus the capacitor discharges for a shorter time, or in other words, to a larger voltage and the time required for the capacitor to recharge to the reference voltage becomes less.