Abstract: A demand cardiac pacer having provisions for determining power source life status both during normal heart operation and during heart stimulation by the pacer. An external magnetic pulse of relatively long duration induces a test mode in the circuit, the pacer reverting to the pacing mode immediately after removal of the magnetic stimulus. When producing pacing pulses in the pacing mode, testing of the patient's heart is facilitated by providing an external magnetic pulse of relatively short duration which is insufficient to place the pacer in the test mode but of sufficient length to inhibit pacing pulses. This invention applies to preset frequency pacers as well as to programmable pacers and includes means for observing the preprogrammed pulse frequency immediately after programming.