Responses of identified single neurons in the isolated right cerebral ganglion of Onchidium to bath applied γ-aminobutyric acid (GABA) and lioresal (LRS) were measured using conventional intracellular microelectrode techniques under current and voltage clamp conditions. In normal medium the neurons responded to both GABA and LRS. GABA induced two successive hyperpolarizations (phase I and II). Phase I was a short-lasting Cl^- permeability increase while phase II was a long-lasting K⁺ permeability increase. LRS evoked only a long-lasting K⁺ permeability increase. In Ca²⁺-free high Mg²⁺ medium GABA produced only short-lasting hyperpolarization while LRS had no effect. GABA-induced phase I responses behaved as a simple Cl^- electrode following changes of external Cl^- concentrations. The Hill coefficient obtained from the relation between the relative change in the GABA current and the GABA concentration was almost one during the current increase phase in Ca²⁺-free high Mg²⁺ medium. These results indicate that GABA has a direct Cl^- permeability increase action on the postsynaptic cell membrane of the identified Onchidium neuron while LRS has no such direct action.