1. Acetylcholine (ACh)-induced currents were studied in completely isolated Lymnaea stagnalis neurones using the voltage-clamp technique. 2. The ACh-activated pathways were shown to be selective for Cl- ions. 3. It was shown that membrane depolarization inhibits ACh-induced conductance. This phenomenon was called 'ACh response inactivation'. 4. Inactivation decreases after lowering the extracellular Ca2+ concentration or after blockade by Mn2+ of the electrically excitable Ca2+ channels. 5. In dialysed neurones an increase of the intracellular Ca2+ concentration inhibits the ACh-induced conductance. 6. The conclusion is made that the inactivation of ACh response by depolarization is initiated by Ca2+ entering the neurone through the electrically excitable Ca channels. 7. The onset and the decay of the ACh response inactivation were studied by analysing the relaxations of the ACh-induced current during and after the application of depolarizing pulses. The most conspicuous relaxation is a slow relaxation observed at the end of a long depolarizing pulse, which appears to reflect the return of the system from the inactivated state to the non-inactivated one. 8. The slow relaxations observed during and after a depolarizing pulse appear correlated with variations of the intracellular Ca2+ concentration, and are distinct from faster relaxations observed in the hyperpolarizing range and attributed to the voltage dependence of the channel open-time.

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