In nine patients (pts.) with recurrent SVT, we have performed a combined electrophysiological and hemodynamic study. Five pts. showed reciprocating nodal tachycardia and 4 pts. reentry tachycardia associated with WPW syndrome (2 with Kent bypass tract and 2 with James bypass tract). Hemodynamic parameters were recorded during sinus rhythm (SR), atrial (AP) and ventricular pacing (VP) and following the initiation of SVT. The arrhythmia was induced by rapid atrial pacing or with atrial or ventricular premature stimuli. Hemodynamic data were then compared. Our results point out a greater hemodynamic deterioration during SVT than during AP or sinus tachycardia at similar rates. We have often observed in SVT, and constantly in A-V nodal reentrant (AVN) cases, the appearance of giant waves in the right and left atrium. This finding confirms hypothesis of the primary role of the changes in atrioventricular contraction sequence during SVT, especially in AVN tachycardias, with subsequent reduction in stroke volume and cardiac output.