The freeze-fracture technique was used to study differences in membrane structure which could explain differences in the number of quanta released from axon terminals on twitch and tonic muscle fibers in Anolis intercostal muscles. The protoplasmic leaflets of axon terminals facing lizard twitch muscle fibers have intramembrane particle specializations characterized by two parallel linear particle arrays each composed of two particle rows which lie perpendicular to the axis of shallow ridges in the axolemma. During K+ depolarization, vesicles open between the arrays, confirming that these structures are the active zones for synaptic vesicle opening. Active zones at axon terminals on tonic fibers are defined by one linear particle array composed of two parallel particle rows oriented along the axis of a shallow presynaptic ridge; vesicles open beside these arrays. Thus, there are more particles near active zone vesicles in terminals on twitch fibers. Even though terminals on twitch and tonic muscle fibers seem to have similar numbers of synaptic vesicles associated with their active zones, a presynaptic action potential is reported to release at least 10 times more quanta from terminals on twitch fibers. We postulate that the differences in quantal output are related to the observed differences in the number of active zone particles flanking synaptic vesicles at the active zone. Indeed, the correlation between the distribution of these particles and the level of transmitter release provides additional support for the idea that they are the calcium channels which couple transmitter release to the action potential.