A cardiac pacer system includes a first relatively large contact area electrode for sensing cardiac activity and a second electrode of substantially smaller contact area than the first to apply artificial stimulating pulses to the heart. The first electrode has a sufficiently large contact area to achieve a relatively high sensitivity to cardiac activity and the second electrode has a sufficiently small contact area to generate a relatively high current density at the electrode-tissue interface when the artificial stimulating pulses are applied to the heart. The electrodes are supported in a spaced relationship by an electrically insulating spacer. Each electrode may be directly wired to an appropriate point within the cardiac pacer, and referenced to a common electrical return point by means of a third, common electrode. Alternatively, the first and second electrodes may be connected to the pacer with a single lead, with suitable decoupling circuitry between the single lead and each electrode. The third or common electrode may be eliminated by having the first and second electrodes each serve as a common electrode for the other, while employing suitable decoupling circuitry between the pacer and the electrodes.