1. Acetylcholine (ACh) noise and miniature end-plate potentials were recorded with focal external micro-electrodes.2. The effect of prostigmine on the time course of the ;molecular' and ;quantal' transmitter actions was studied. Prostigmine (10(-6) g/ml.) has little or no effect on the duration of the molecular ;gating action', while it greatly prolongs the quantal conductance change.3. After inhibition of ACh hydrolysis, the removal of the transmitter from the synapse is generally too slow to be accounted for by free diffusion. It is suggested that diffusion is delayed by binding to post-synaptic receptors. This is consistent with the finding that receptor blockage by curare or alpha-bungarotoxin shortens as well as reduces quantal transmitter action.4. The correlated effects of the receptor-blocking agents, on size and time course of the miniature end-plate currents, were subjected to a simple analysis. Its result suggests that after inhibition of cholinesterase about two thrids of the quantal packet of ACh combines with post-synaptic receptors.5. During focal external recording the effect of prostigmine on the time course of miniature end-plate potentials can become exaggerated due to what appears to be a compression artifact which obstructs outward diffusion of the transmitter.