An unusual case is presented in which ventricular tachycardia at a rate of 141 beats/min was transiently entrained by rapid atrial pacing at rates of 150, 155 and 160 beats/min, and was interrupted by rapid atrial pacing at a rate of 165 beats/min. During each period of transient entrainment, constant ventricular fusion beats were present except for the last entrained beat, and progressive ventricular fusion (different fusion QRS complexes) was demonstrated when comparing QRS complex configurations during transient entrainment at each pacing rate. Interruption of the ventricular tachycardia was associated with localized conduction block to the right ventricular recording site, followed by activation of that site from a different direction and with a shorter conduction time by the subsequent pacing impulse. These data clearly distinguish transient entrainment of a tachycardia from overdrive suppression of a tachycardia, and strongly suggest that reentry was the underlying mechanism of the ventricular tachycardia.