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1. The `local vesicle population', namely the population of synaptic vesicles lying within a zone 0·25 µm wide adjacent to the presynaptic membrane and expressed as `vesicles µm^-2' has been estimated in rat superior cervical ganglia maintained in vitro for 1 h. Ganglia were either unstimulated or stimulated tetanically during the last few minutes of the in vitro period.

2. Individual local vesicle populations at synapses in groups of ganglia which had received the same treatment showed an approximately normal frequency distribution.

3. The technique of stimulation did not permit fixation during tetanus, but the mean local vesicle population in ganglia fixed a few seconds after 10 Hz tetani of up to 10 min duration were significantly (P < 0·001) higher than the unstimulated control value of 124·0 ± S.E. 3·2. Ganglia fixed a few seconds after 3 min tetani at 10 Hz had a mean local vesicle population of 155·7 ± S.E. 5·7, rising slightly but not significantly to 165·0 ± S.E. 4·3 at 1 min and 163·1 ± S.E. 5·2 at 2 min after tetanus. Thereafter the mean local vesicle population fell slowly, reaching control level between 10 and 20 min after tetanus.

4. In terms of the vesicle hypothesis, the observations suggest that post-tetanic potentiation might arise from a closer clustering of vesicles about release areas resulting in an increase in fractional release. It is suggested that stimulation increased the local vesicle population by promoting vesicle mobilization rather than by altering the size or shape of the presynaptic terminals. The `mobilizing agent' might be intracellular Ca²⁺.

5. The results appear to be consistent with those of other workers where experimental conditions and the region in which vesicles were counted were comparable.

6. Alternative interpretations for the findings, not involving the vesicle hypothesis, are considered.