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.