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1. The membrane conductance of the crayfish muscle fibre was calculated and the increase in the membrane conductance brought about by the application of γ-aminobutyric acid (GABA) was investigated under various conditions. The permeability of the activated inhibitory post-synaptic membrane was estimated from the conductance change.

2. When the concentration of Cl was changed by substituting Cl with methylsulphate, a linear relation was obtained between the membrane conductance change produced by GABA and the concentration of Cl. Cations have no appreciable effect on the GABA induced conductance change (less than 0·4%).

3. The conductance change produced by GABA on the inhibitory post-synaptic membrane was measured in various solutions in which the Cl was replaced by various anions. The degree of permeability depended on the anion species in the following order:

Br > Cl > SCN > I > NO3 > HCOO > ClO3 > ClO4 > BrO3

4. At high pH the conductance change produced by GABA was decreased, while at low pH it increased.

5. These results show that the activated inhibitory post-synaptic membrane of crayfish muscle fibres has the characteristics of an anion selective membrane, suggesting the fixed positive charges in the inhibitory membrane.