Abstract: A cardiac pacemaker circuit for use with a stimulating electrode embedded in a patient's atrium or ventricle and a reference electrode positioned outside the heart or on the same lead within the heart. Stimulus pulses are applied by the circuit across the two electrodes, these stimulus pulses replacing evoked pulses. On termination of each stimulus pulse the amount of charge stored between the two electrodes is monitored and pulses of short duration are applied to the stimulating electrode progressively to reduce this charge over a very short time period to a level at which the intrinsic pulses of the evoked heart response can be monitored. The evoked heart response can then be used for diagnosis while the pacemaker circuit is in operation. If the stored charge was not reduced quickly then the charge would mask the intrinsic pulse and the pacemaker would have to be switched off before the intrinsic pulses could be monitored for diagnosis.