The development of intelligent and adaptive cardiac pacemakers is limited by the absence of a reliable measure of physiological state. Evidence exists to suggest that the required information may be obtained from the paced-evoked cardiac potentials. The most convenient way to measure these potentials, in clinical practice, is to use the existing single unipolar electrode for both stimulation and measurement. Owing to the electrochemistry of the electrode tissue system, an afterpotential exists across the electrodes following the stimulus. This is approximately two orders of magnitude larger than the potential to be measured. The system developed by the authors and described here cancels the after-potential in a novel, quasi-closed-loop, adaptive manner. The designs of the system and the electronic circuitry are described. Several sets of equipment have been constructed and used by the authors, and others, for the reliable measurement of the paced evoked cardiac potential.