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It was discovered by Coombs, Eccles & Fatt (1955a) that the hyperpolarization of the inhibitory post-synaptic potential (IPSP) changed to a depolarization when Cl-ions were injected either by diffusion or electrophoretically into cat spinal motoneurones. This effect was satisfactorfly explained by assuming that, when activated, the inhibitory post-synaptic membrane is permeable to C1-ions. A similar effect was obtained for Br-, NO3-and SCN-ions, but not for HC03-, CH3C02-, SO42m HPO and glutamate ions. Since the ions in the former group are smaller in their hydrated form than those in the latter, it was postulated that in the activated state the inhibitory post-synaptic membrane has a sieve-like structure with a pore size large enough to allow the passage of SON-ions, yet small enough to block bicarbonate and acetate ions. A differential permeability of the inhibitory post-junctional membrane was also reported on crustacean muscle (Boistel & Fatt, 1958) and crustacean stretch receptor cell (Hagiwara, Kusano & Saito, 1960) for C0-ions on the one hand, there being impermeability to sulphate and large organic ions on the other. The first aim of the present investigation was to employ a more extensive anion series in order to test the postulate that the differential permeability is explicable by the pore size. Eleven anion species have now been established as penetrating and another sixteen as non-penetrating through the activated inhibitory post-synaptic membrane of cat spinal motoneurones. A preliminary report of some of these findings has been given (Araki, Ito & Oscarsson, 1961). Coombs et al. (1955a) observed that the size of the IPSP, changed by injection of Cl-, Br-, NO3-and SCN-ions, recovered exponentially with a time constant of about 30 sec. In the course of the present experiments it was found that the time course of the IPSP recovery varied according to the injected anion species. Since the recovery is caused by the removal * Rockefeller Fellow. Present address:

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