1. Miniature end-plate potentials (min.e.p.p.s) were recorded intracellularly from frog neuromuscular junctions.2. The ;phasic' release of transmitter which is directly related to nerve impulses was suppressed by withdrawal of Ca from the external medium plus addition of Mg.3. Under these conditions, min.e.p.p.s continued to be discharged even when EGTA was added, although in this case min.e.p.p. frequency appeared to decrease to about half the rate in normal Ringer.4. Tetanic stimulation of the nerve approximately doubled the rate of min.e.p.p.s even in Ca-free solutions with EGTA added.5. The tetanic increase in frequency was greater without EGTA and greater still with some Ca added. Therefore, it is concluded that the tetanic rise in min.e.p.p. frequency can occur even in the absence of the immediate ;phasic' release of transmitter normally induced by nerve impulses; and that the magnitude of the increase is related to Ca concentration.A possible relation between ;phasic' and ;residual' effects of nerve impulses is described.