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Previous reports from this laboratory described an electrotonic mechanism for simple impulse transmission through blocked segments of canine Purkinje tissue with slow diastolic depolarization assuming a vital role in second degree block. Utilizing an electrical blocking current, a blocked segment of canine Purkinje tissue was produced. Transmembrane events were recorded from blocked segments during higher grades of block (3:1 to complete) to delineate further mechanisms responsible for a periodic distal boundary response. Our results confirm that slow diastolic depolarization is an important determinant in sustaining periodic impulse conduction. Its importance is related to (1) progressive decrease of the resting membrane potential toward threshold at the distal block boundary, and (2) augmentation of the transmitted electrotonic potential in accordance with voltage dependent changes in membrane resistance. These data further lend definition to the distinction between electrotonic, partially active, and active transmembrane potentials. Impulse transmission through a segment of inactivated tissue is electrotonic and slow diastolic depolarization plays an important role in the maintenance of periodic impulse transmission.