A computer simulation model of cardiac rhythm and heart-pacemaker interactions has been used to examine the response of short AV delay DDD pacing to an atrial premature depolarization in patients with a potential junctional reentry circuit. Special attention was given to the possibility of using the pacemaker to prevent atrioventricular reentry tachycardia mediated by a bidirectional accessory pathway. The computational experiments examined: (a) the differences between pacing modes with constant and quasi-Wenckebach prolongation of AV delay; (b) the effects of imposing the restrictions of a pacemaker atrial refractory period and upper rate limit; and (c) the effects of varying the ventricular refractory period so that it was either the same or longer than that of the atria. The computational results prove that some possible modes of initiation of junctional reentry tachycardia are beyond the prophylactic capabilities of current DDD pacemakers. Future possibilities involving the use of computer modeling to develop more sophisticated pacemaker modes are briefly discussed.