Spread of excitation in isolated canine bundle branches was examined by microelectrode technique in order to investigate the possibility of dissociated impulse transmission in normal Purkinje fibers. In all experiments, local excitation of the preparation evoked either by fine bipolar extracellular electrodes or an intracellular microelectrode proceeded much faster along the longitudinal axis of the fibers than along the transverse axis. As a result, the spread of excitation in the vicinity of stimulating site showed significantly inhomogeneous character. The inhomogeneity of excitation spread became more manifest with more eccentric location of the stimulating site in a given preparation. Larger preparation showed greater degree of inhomogeneity. Nevertheless, the inhomogeneous spread of excitation alone appeared unlikely to provide multiply pathways which were functionally dissociated with each other. When premature stimulation was applied, however, different conductivity among fibers within a single false tendon was oberved, suggesting that each conducting element became more independent.