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1. X-537A is an ionophore that can carry cations across cell membranes. We studied its effects on spontaneous and stimulated quantal acetylcholine (ACh) release at the frog neuromuscular junction. 2. When neuromuscular transmission was blocked with high Mg2+ or with curare, X-537A markedly increased the end-plate potential (e.p.p.) amplitude. Then a few minutes later the e.p.p. disappeared. 3. When neuromuscular transmission was blocked with hypertonic saline solution, X-537A did not increase e.p.p. amplitude; it did produce many transmission failures. 4. X-537A decreased the depolarization of the end-plate produced by iontophoretically applied ACh. this may account in part for the disappearance of the e.p.p. in solutions containing the ionophore. 5. X-537A depolarized muscle fibres by about 15 mV. 6. When the extracellular divalent cation concentration was very low, X-537A had little or no effect on miniature end-plate potential (min.e.p.p.) frequency. 7. When a divalent cation was present in the extracellular fluid, X-537A increased the frequency of the min.e.p.p.s. The sequence of effectiveness of the divalent ions we have tested is: Ba2+ greater than Sr2+ greater than Ca2+ greater than Mn2+ congruent to Co2+ congruent to Ni2+ greater than Mg2+. There is a rough parallel between these results and the reported affinity of X-537A for various divalent ions. 8. The increase in min.e.p.p. frequency caused by X-537A was transitory, following the increase min.e.p.p. frequency fell to a very low rate or to zero. Then nerve stimulation did not cause quantal release. A second application of X-537A was without effect. 9. X-537A decreased min.e.p.p. amplitude, in accord with the effect on the sensitivity of the end-plate to ACh. 10. The results support the idea that increases in intracellular divalent cation concentrations trigger quantal release from nerve terminals and are involved in the disensitization of end plate receptors to ACh.