Isolated canine interventricular septa were studied with standard microelectrode tech-niques. Focal automaticity was induced by applying depolarizing current through an extracellular pipet in contact with the right bundle branch (RBB) of the ventricular specialized conducting system. Automaticity appeared with depolarization to transmembrane potentials of −50 mV or less. The spontaneous activity was neither depressed nor accelerated when overdrive suppression was at-tempted. Activity originating within the focus propagated into fully polarized surrounding tissue. However, entrance block, phasically related to the spontaneous cycle length, was an intrinsic property of these foci. Early premature beats initiated outside the focus failed to enter the focus, but the resulting electrotonus delayed the next automatic beat. Late premature beats captured and thereby accelerated the focus. Thus, the automatic foci could be entrained by extrafocal activity. Consequently, continuous pacing at various rates precipitated complex rhythms with fixed coupling. Similar foci with exit conduction, entrance block, and electrotonic modulation also were demonstrated in focally depolarized papillary muscles in feline septal preparations. The unique properties of focally depolarized areas in which spontaneous activity is generated at low membrane potentials provide a mechanism capable of generating a wide array of arrhythmias.Circ Res 47: 238-248, 1980