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1. Synaptic potentials were recorded with intracellular electrodes from cells in the inferior mesenteric ganglion of the guinea-pig.

2. Half-widths of the synaptic potentials recorded fell into two groups: type L cells had long synaptic potentials (11·6-15·2 msec) and low thresholds (14·6 mV mean), type S cells had short synaptic potentials (6·1-9·3 msec) and high thresholds (29·9 mV mean).

3. Physostigmine (1·2 x 10^-6 M) caused a significant increase in the half-width of both types of synaptic potential.

4. Physostigmine caused a significant increase in the half-width of spontaneous synaptic potentials and an increase in their amplitude.

5. Repetitive preganglionic stimulation, in the presence of physostigmine, led to a marked and prolonged depolarization in all cells. In most cells repetitive spontaneous firing of action potentials was then observed. This effect was blocked by atropine (1·4 x 10^-7 M).

6. The effect of atropine on the half-width in a physostigmine-treated cell was inconsistent: although synaptic potentials in some cells were slightly shortened their half-widths were always greater than the control.

7. It is concluded that cholinesterase plays a role in limiting the time course of the synaptic potential, by limiting the duration of action of acetylcholine.