1. The mechanism of procaine action on post-synaptic receptors for acetylcholine was studied by recording the end-plate current at membrane potentials ranging from about +30 to about -140 mV.2. It has been found that at resting membrane potential of about -60 to -80 mV the end-plate current has a fast initial and a slow late component. During hyperpolarization of the muscle fibre the amplitude of the slow component is depressed and its half-time lengthened. When the membrane potential is inverted the difference in the time course of both components is much less pronounced or absent.3. It is suggested that procaine modifies the receptor response induced by acetylcholine, and that this modification is dependent on membrane potential.