Evidence for the vesicle hypothesis.

1. The relationship of synaptic vesicles to the synaptic cleft was examined with the electron microscope at neuromuscular junctions in the rat diaphragm before and after bathing the preparation in a physiological salt solution for 2 hr.2. A population of vesicles was defined which appeared to ;touch' the axoplasmic membrane. These vesicles were found to be aggregated adjacent to axoplasmic densities which lay opposite the mouths of post-synaptic junctional folds.3. Soaking in the salt solution and modifications of this solution increased the proportion of folds opposed by presynaptic densities with associated vesicles.4. Soaking in solutions with 20 mM-KCl depleted both the specific vesicle population and the whole population of terminal vesicles. The effect was shown in paired experiments to be a specific effect of the 20 mM-KCl, and it was prevented by a concomitant increase of the bathing MgCl(2) concentration.5. Soaking in solutions with a raised osmotic pressure reduced the specific but not the general vesicle population.6. It is suggested that these observations support the vesicle hypothesis and that the specific vesicle population forms part of a feed-back mechanism adjusting transmitter synthesis and mobilization to the rate of release of transmitter.