Recordings of synaptic currents from the crayfish opener muscle were made with a macro-patch recording technique, permitting clear detection of neurotransmitter quanta at individual nerve terminals before and after induction of long-term facilitation (LTF). Depolarization of the terminal by propagated action potentials or by local intracellular pulses induced LTF. The quantal content was increased on average by 93%. Binomial analysis indicated increased probability of release and also increased number of available quantal units. The increase occurred regardless of a blockade of sodium, calcium or potassium channels by appropriate pharmacological agents. Presynaptic recording with an intracellular microelectrode showed no change in presynaptic electrical properties. Also, there were no changes in the synaptic delay. It is concluded that LTF results from a depolarization-dependent alteration of synaptic release sites.