1. The procaine e.p.p. has a special hump on its falling phase. Various properties of procaine e.p.p. were analysed by the technique of intracellular recordings.
2. When the initial membrane potential was displaced artificially, the size of the e.p.p. and that of the hump changed in opposite directions, i.e., the hump became more marked when the membrane potential was lowered, and decreased when the latter was high. At the membrane potential of about100mV, the hump was totally abolished.
3. Nature of the procaine hump of the e.p.p. was discussed. Probably, an augmented response of the end-plate at the later stage of the transmitter action and a local response of the muscle fiber are concerned to the formation of the hump.
4. Procaine decreases the sensitivity of the end-plate to ACh externally applied, 10^-4 procaine being of approximately equal potency to 1-2×10^-6 tubocurarine. This seems to be the most important factor for the neuromuscular block produced by procaine.