Motor nerve terminals in cutaneous pectoris muscles of the frog Rana pipiens release more transmitter and form synapses with higher levels of effectiveness than do those in sartorius muscles. Neuromuscular junctions from these two muscles were compared in the electron microscope to search for ultrastructural correlates of differences in transmitter release and synaptic effectiveness. The following measurements were made from cross-sections of junctions with known levels of effectiveness: (a) the presence of active zones, the presumed sites of transmitter release, (b) active zone size, (c) the perimeter, cross-sectional area, height and width of nerve terminals, (d) number of mitochondria, (e) vesicle density, and (f) the extent to which Schwann cells wrap terminals. Nerve terminals in the two muscles did not differ in size, shape or vesicle density. The more strongly releasing cutaneous pectoris terminals did, however, have significantly larger active zones due to deeper invagination of the terminal into the postsynaptic gutter and lesser interposition of Schwann cell processes between presynaptic and postsynaptic membranes. Cutaneous pectoris terminals also contained more mitochondria, presumably to supply the greater energy demand imposed by high release levels.