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1. An analysis has been made of the release of acetylcholine (ACh) from the preganglionic nerve terminals of the isolated superior cervical ganglion of the guinea-pig during prolonged repetitive stimulation (10 Hz), using the amplitude of the excitatory post-synaptic potential (e.p.s.p.) as a measure of the amount of ACh released. 2. The decline in the mean amount of ACh released by each impulse over 20 min of continuous stimulation was accompanied by a small decrease in the mean miniature (min.) e.p.s.p. amplitude (less than 20%), both in the presence and absence of supplementary choline (3 x 10(-5) M). During stimulation in the presence of hemicholinium-3 (HC-3) (2 x 10(-5) M), the fall in min.e.p.s.p. amplitude was significantly larger. 3. Amplitude-frequency histograms of e.p.s.p.s evoked at different times after the beginning of stimulation were usually well predicted by binomial statistics, using the min.e.p.s.p.s released during each sample period as a measure of the quantal unit. In the other cases, histograms could be predicted using Poisson's Law. 4. A decline in quantal content, m, occurred in all experiments. In the presence of synthesis, with or without added choline, this was always associated with a decrease in the binomial parameter, n, and, in many cases, with a decrease in the binomial parameter, p. During stimulation in the presence of HC-3, a larger fall in p was observed in all experiments. 5. The results suggest that depletion of the ACh stored in the terminal decreases both the size and number of quanta available for release, and that the decrease in the amount of ACh in each quantum reduces the probability of its release.

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