The nature of the falling phase of the synaptic current was studied in more detail at the crayfish neuromuscular junction and fundamental data relating to focally recorded excitatory junctional potentials (e.j.p.s) was complemented. The decay of extracellular e.j.p.s at the crayfish neuromuscular junction was almost exponential. The decay time constant of extracellular e.j.p.s varied considerably from one e.j.p. to another, the mean of the decay time constant being about 1 ms at the normal resting potential at a bath temperature of 22 degrees C. There was no detectable correlation between the decay time constant and the amplitude of extracellular e.j.p.s which were induced by a single stimulation of the excitatory axon. Prolonged application of glutamate hardly affected the decay time constant of extracellular e.j.p.s. The significance of the study on the crayfish neuromuscular system was discussed.