Synaptic terminals on branches of an excitatory motor axon in a spider crab (Hyas areneas) were examined by electron microscopy to determine whether differences in size, structure, and number of synapses could be correlated with differences in transmitter release. Terminals releasing relatively large amounts of transmitter during low frequencies of nerve impulses ("high-output" terminals) had larger synapses, more prominent presynaptic dense bodies (active zones), and fewer synapses per unit length than terminals releasing relatively small amounts of transmitter ("low-output" terminals). Neither the difference in synaptic area, nor the quantitative differences in the active zones, were sufficient in themselves to explain the difference in synaptic efficacy, and it is postulated that a non-linear relationship may exist between structural features of the synapse and release of transmitter by a nerve impulse, and that differences other than those apparent from the ultrastructure could be involved. Greater facilitation at low-output terminals with high frequencies of nerve impulses may be due to greater reserves of "immediately available" transmitter, and to recruitment or activation of more individual synaptic contacts.