Ventricular-programmed pacemakers are designed to avoid the possibility of dangerous or lethal arrhythmias provoked by stimulation during the vulnerable phase of the ventricle. Even so, rapid heart action (tachyarrhythmia) may still occur. Illustrative examples are presented in which tachyarrhythmia is presumed to have occurred or in which the vulnerable phase has been stimulated, because of: 1)inferior electrode placement, 2) the interaction of a unit with certain characteristics and spontaneous premature beats, 3)a long Q-T interval and late vulnerable phase, 4)failure to pace at adequate rates, and 5)an unusual ``coupled'' extrasystolic arrhythmia.