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1. The membrane conductance of the crayfish muscle was measured with intracellular micro-electrodes. Increase in the membrane conductance induced by various concentrations of -amino butyric acid (GABA) was measured in the Br^-, NO₃^-, I^- and CNS^- solutions and compared with that in Cl^- solution. When the membrane was activated by lower concentrations of GABA, the resulting membrane conductance in NO₃^- and I^- solutions was larger than in Cl^- solution, while the membrane conductance activated by higher concentrations in NO₃^- and I^- solutions were smaller than in Cl^- solution. The membrane conductance activated by GABA was larger in Br^- solution and smaller in CNS^- solution than the membrane conductance activated in Cl^- solution, throughout the concentration range of GABA examined.

2. The reversal potential of inhibitory junctional potentials (i.j.p.s) was measured shortly after replacing Cl^- by foreign anions: Br^-, NO₃^-, I^- and CNS^-. In I^- solution the i.j.p.s produced by lower stimulation frequencies showed a reversal potential at a more hyperpolarized level than those produced by higher stimulation rates.

3. Shortly after replacing Cl^- by I^-, the GABA potential induced by iontophoretic application showed a triphasic potential change at the `reversal' potential. The reversal level of the GABA potential produced by smaller doses was at a more hyperpolarized level than that produced by larger doses.

4. Possibilities of a concentration effect of GABA are discussed. It is suggested that the relative permeability of the crayfish inhibitory membrane to anions changes depending on the concentration of GABA.