Abstract: A pacemaker of the inhibited demand type which is equipped with a battery and pacemaker efficiency evaluator. A magnetically initialized test circuit synchronizes the timing of test stimulating pulses with the natural R-wave or, the end of a basic pacemaker escape interval, thereby to enhance patient safety by avoiding competition during testing. The synchronized test circuit produces an alternating sequence of sensing and pacing periods which are maintained during the test. Upon actuation of the test, the first R-wave, or the end of the basic pacer interval, is sensed and a number of stimulating pulses is applied at the end of successive test intervals, which intervals are located between pacemaker output pulses, the duration of which indicates the battery status. This sequence of sensing, synchronization, an application of test pulses is repeated during the duration of the magnet test.