Preemptive tachyarrhythmia pacing is provided in an implantable cardiac-stimulation device, such as an implantable pacemaker or defibrillator, by modifying the operation of the implantable device in a way that minimizes the likelihood of occurrence of a tachyarrhythmia. The behavior modification is achieved through the use of an appropriate preemptive tachyarrhythmia pacing control routine stored within the memory of the device. Depending upon the needs of the patient, preemptive tachyarrhythmia pacing is invoked continuously or on demand. If invoked on demand, Preemptive tachyarrhythmia pacing is triggered only upon the sensing of one or more conditions suggest that the onset of a tachyarrhythmia is imminent. When thus invoked, preemptive tachyarrhythmia pacing remains invoked only for as long as the onset-of-a-tachyarrhythmia-is-imminent conditions persist. Various preemptive tachyarrhythmia pacing control routines are contemplated, including those based on overdrive pacing, pacing with randomicity, and mode switching. Overdrive pacing, when used, is based on an automatically-determined diurnal rate, an automatic stepped increase over the average atrial rate, or a rate set by a cycle-to-cycle negative hysteresis. Conditions which suggest the imminency of the onset of a tachyarrhythmia include, but are not limited to, a sudden change in the patient's cardiac cycle from a previously-determined normal cardiac cycle.