Excitatory postsynaptic currents (EPSCs) were recorded extracellularly from synaptic spots on crayfish opener muscle fibers. Synapses on the proximal fiber bundle were characterized as fast, with a relatively high quantal-release rate m of 0.2-5 and a low twin-pulse facilitation Fs of 1.1-3, at 13.5 mM [Ca]o and low (0.5/s) repetition rate. Under the same conditions, distal "slow" synapses had a release rate m of 0.02-0.4 and a facilitation Fs of 2-4. When the [Ca]o was varied between 1.7 and 27 mM, release and facilitation were much less affected in proximal, fast synapses than in distal, slow ones. The average maximal slope of the log release to log [Ca]o relation was 1.5 in proximal, and 3.1 in distal synapses, while the average maximal facilitation Fs was 2.5 in proximal and 4.7 in distal synapses, respectively. Assuming saturation kinetics for entry of Ca into the terminal and release of transmitter, possible variations of parameters generating the fast-slow differentiation were explored. Excluding a number of possibilities, it was found that in addition to a higher maximal release level, fast synapses seem to have a higher resting [Ca]i and/or a lower cooperativity of the release mechanism, as compared to slow synapses.