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1. 1. The effects of acidic pH on transmitter release were studied at the frog neuromuscular junction, using intracellular recording techniques. 2. Acid pH reduced the amplitude of the end-plate potentials (e.p.p.s) and accelerated the frequency of the miniature e.p.p.s(m.e.p.p.s). 3. At pH 6-0 the m.e.p.p. frequency was on the average 2-5 times greater than at pH 7-4. This multiplication was independent of the divalent ion concentration of the medium over a large range. 4. Reduction of the e.p.p. amplitude at low pH was the result of a decrease in m, the number of quanta of transmitter liberated by the nerve impulse. 5. The effect of low pH on m was blocked by high concentrations of Mg2+ and by lower concentrations of Mn2+ ions. This occlusion was found even when the total concentration of divalents in the bathing solution was kept constant. 6. These results indicated that H+ and Mn2+ ions bind to an acidic site which regulates Ca-mediated release of acetylcholine (ACh). The acid dissociation constant (KH) was determined using both a kinetic and a surface charge model. The pKa of the site calculated from the kinetic model was 5-7, while a pKa of 3-6 was obtained from the surface charge model. 7. It is suggested that protonation of the acidic site mentioned above reduces evoked transmitter release by blocking the influx of Ca into the nerve terminal following the nerve action potential.