Transmembrane activity was recorded from canine false tendons bathed with Tyrode's solution at 37°C. Stimulus patterns provided a 3-second pause after every ten beats. Acetylstrophanthidin was infused at concentrations up to 2x10^-7 g/ml. One or two transient depolarizations (TDs) followed the last driven response of each series. The appearance of TDs was associated with depression of normal phase-4 depolarization. The peak of the earliest TD (TD-1) occurred at an interval approximately equal to the basic cycle length. The later TD (TD-2) occurred at about twice the basic cycle length. Coupling intervals were determined primarily by the last cycle length. The amplitude of TD-1 was maximal when the basic cycle length was 600 msec, but TD-2 continued to increase as the basic cycle length diminished further. The amplitude of both TDs increased with the number of beats in the train. Either or both could reach threshold and induce single extrasystoles or trains of extrasystoles. TDs could be induced to reach threshold after each driven response, resulting in sustained bigeminal rhythms with fixed coupling. Possibly TDs provide a mechanism for various clinically observed arrhythmias induced by cardiac glycosides.