A patient with recurrent bouts of atrial fibrillation and wide-complex regular tachycardia underwent electrophysiologic studies. Premature atrial stimulation or atrial pacing during sinus rhythm resulted in gradual lengthening of the PR and AH intervals, narrowing of the HV interval and progressive preexcitation with a left bundle branch block and left-axis contour. Induction of tachycardia was dependent on critical delay in the atrioventricular interval and was associated with attainment of a maximal preexcitation pattern. During tachycardia, the ventriculoatrial interval was constant, whereas the interval from His bundle deflection to the ventricular complex was variable. We postulate that the tachycardia circuit involved reciprocation within the atrioventricular node and that a nodoventricular bypass tract was present in close anatomic or functional association with the slow atrioventricular nodal pathway. Our data suggest that both the nodoventricular bypass tract and the His-Purkinje system may be passive "bystanders" rather than essential components of the tachycardia circuit. In addition, although HV dissociation usually implies ventricular tachycardia, this case demonstrates that HV dissociation during wide-complex regular tachyarrhythmia is not diagnostic of ventricular tachycardia.