The effect of temperature on the form of the propagated action potential was investigated in frog skeletal muscle fibres. Increasing the temperature decreased the duration of the initial overshoot but a hump then appeared during a more prominent after-depolarization. Finally, at 28-30 degrees C, the after-depolarization was either noticeably enlarged or entirely absent. This all-or-none failure of tubular conduction suggests that excitation of the tubular membrane takes place through regenerative activity rather than graded electrotonic spread of depolarization. However, it is consistent with a partial electrical isolation of the tubular lumina, possibly through the access resistance proposed in earlier theoretical models for muscle membrane.