1. Synaptic facilitation was measured with intracellular recording at two classes of neuromuscular synapses in the opener muscle of the crayfish dactyl by placing a test stimulus at various intervals after either a single conditioning stimulus or a short conditioning train.2. The facilitative effect of one stimulus reaches approximately the same level with both the superficial central and superficial distal synapses. The facilitation decreases smoothly in two phases after the conditioning stimulus at superficial central synapses and in a more complex fashion at superficial distal synapses.3. The two synaptic types differ in the manner in which they add up the facilitative effects produced by each of the stimuli in a short train. With superficial distal synapses the facilitative effects of conditioning stimuli add linearly, while with superficial central synapses the facilitative effects accumulate exponentially.4. The linear addition of facilitation at superficial distal synapses is not altered when the quantal content is lowered by decreasing the external Ca concentration from 13.5 to 3 mM.5. The rate of decay of facilitation is the same following both one and three conditioning stimuli, even though the facilitation is nearly six times larger in the latter case.6. The results are discussed in terms of mechanisms for synaptic facilitation.