1. A demand cardiac pacer having a pulse generator operating at a pulse interval determined by the programming of said pacer when it is in the asynchronous mode and by received QRS signals when it is in the demand mode, and comprising: a. means for sensing the presence and absence of an externally applied magnetic signal; b. first means for counting said pulse intervals when said external magnetic signal is sensed; c. second means for counting said pulse intervals when the absence of said external magnetic signal is sensed; d. control signal means for generating control signals as a function of the counts of said first and second counting means; and e. control means for controlling at least one predetermined pacer parameter with said generated control signals.