Previous work has characterized the Purkinje-ventricular junction (PVJ) of papillary muscles as a region of low safety factor with some delay between P and V activation. We have previously shown that the P strand action potential activates a layer of P cells over most of the papillary muscle surface, but activation of the underlying V cells occurs only at specific junctional sites, generally near the base of the muscle. Our present results from both dog and rabbit left papillary muscles show that the junctional regions for propagation from the V layer up into the P layer include not only these specific P-to-V sites but also other sites located more toward the apex of the muscle. These results show that the papillary muscles have regions of potential unidirectional block, although the manifestation of unidirectional block at these sites requires the premature excitation of the ventricular layer.