An apparatus for stimulating and sensing evoked response to stimulus in the heart. First and second electrodes are in electrical contact with the heart, a third indifferent electrode is also in electrical contact with the heart. A pacemaker provides stimulus signals through the electrodes in the stimulating mode of operation. The first and second electrodes are switched through switching apparatus wherein in the first mode the first and second electrodes are maintained at equal electrical potentials, and in a second, sensing mode, the switch operates between the first and second electrodes so as to allow the first and second electrodes to act as bipolar sensing leads. Evoked response is sensed by a differential amplifier having a first differential input connected to the first electrode and a second differential input connected to the second electrode. The differential amplifier provides a differential signal which is proportional to the evoked cardiac response.