1. A statistical analysis has been made of the changes in the binomial parameters n and p during transmitter release from motor-nerve terminals when synapses are forming in either regenerating or reinnervated mammalian muscle.2. The amplitude-frequency distribution of e.p.p.s evoked by a single impulse was well predicted by binomial statistics at all junctions analysed allowing values of n and p to be determined. At most of these junctions, p > 0.5 and Poisson statistics did not predict the observed distributions at all.3. During the changes in quantal content which occur during short trains of impulses or during continual stimulation over minutes, no significant change in p was detected. Changes in quantal content of the e.p.p. were therefore determined by n.