1. When L-glutamate and L-aspartate are simultaneously applied to the excitatory neuromuscular junctions of Maia squinado, they produce an increase in the conductance of the post-synaptic membrane much larger than the sum of conductance effects produced by the individual amino acids alone. 2. An examination of the synaptic noise occurring during this synergistic action reveals that the elementary conductnace events produced by aspartate are suppressed and that normal elementary conductance events produced by glutamate are occurring at an enormously increased rate. 3. It is suggested that aspartate causes this potentiation by inhibiting a system for transmitter inactivation in the region of the post-synaptic receptors and that this system, under normal conditions, prevents the access of externally applied glutamate to the synaptic receptors.