1. In neurones BL and BR 3, 6, 8, 9, 10 and 11 of Aplysia buccal ganglia, cholinergic inhibitory post-synaptic potentials are produced by activity in either of two presynaptic cells. In order to analyse the synaptic conductance change, neurones were voltage-clamped inhibitory post-synaptic currents (i.p.s.c.) recorded. 2. The synaptic conductance change rises to an average peak value of 0.65 micromho and decays exponentially with single time constant tau of 19 msec. 3. We have attempted to identify the rate-limiting step responsible for i.p.s.c. decay from among the following possibilities: (1) acetylcholine (ACh) supply, (2) ACh removal by diffusion, (3) ACh removal by hydrolysis or (4) a slow unbinding or conformational change closing open synaptic current channels. 4. Cooling prolongs tau, with Q10 of 5.2. Cooling and eserine treatment together produce greatly prolonged, exponentially decaying i.p.s.c.s with tau > 150 msec. These results suggest that ACh removal, either by diffusion or hydrolysis, is not the rate-limiting step. 5. Prolonging synaptic action potential time course with intracellular injection of tetraethylammonium broadens the i.p.s.c. peak but does not affect the decay tail, suggesting that the rate-limiting step is not ACh release. 6. The spectrum of ACh-induced current fluctuations is fitted by a double Lorentzian with cut-off frequencies of 7.8 and 47 Hz. The frequency of the slower component corresponds to the macroscopic i.p.s.c. decay tau. 7. We conclude that a slow conformational change closing open synaptic current channels is likely to determine i.p.s.c. decay. We cannot, however, exclude either delayed diffusion or a late tail of slow ACh release as possibilities.