The effects of pH and curare on the time course of end-plate currents at the neuromuscular junction of the frog.

1. The effect of pH changes on synaptic currents has been analysed by external recording of the miniature end-plate currents (m.e.p.c.s) or by recording in voltage-clamped end-plates the current elicited by nerve stimulation (e.p.c.). 2. Changes in pH do not appreciably effect the peak amplitude of the current produced by a single quantum or by short ionophoretic pulses of acetylcholine. 3. The time constant of decay of the m.e.p.c.s is prolonged by about 50% in acid pH and shortened by about the same amount in alkaline pH. This effect is independent of the cholinesterase activity of the end-plate. 4. In curarized preparations the decay of the e.p.c. is shorter than in Mg-blocked end-plate even in the absence of cholinesterase blocking agents. 5. The action of pH on the decays can be explained by a titration of the surface charges of the membrane which effects the voltage dependent reaction that controls the rate of closing of the synaptic channels.