Seven cases of tachycardia-dependent, or phase-3, intraventricular block have been examined. Analysis revealed a constant overlap between the range of the R-R intervals ending in normal conduction and the range of the R-R intervals where the second beat is associated with intraventricular block. The block, indeed, may occur at the end of relatively long R-R intervals, whereas R-R intervals which are shorter (up to 0.11 sec) can unexpectedly result in normal intraventricular conduction. A relatively late QRS complex, however, can reflect an intraventricular block only when the preceding complex also manifests the block. This phenomenon has been interpreted as due to the so called "linking", namely the retrograde concealed penetration of an anterogradely blocked bundle branch by the impulse traversing the controlateral bundle branch. This delays the activation of the affected bundle branch, whose refractory period is, accordingly, "shifted to the right" within the cardiac cycle. A relatively late sinus impulse, thus, can result in intraventricular block since the refractory period of the affected bundle branch ends later, with respect to the beginning of the QRS complex, than it does after a normally conducted sinus impulse.