GABA, 3 of its derivatives (l-GABOB, [1-.gamma.-aminohydroxybutyric acid] d-GABOB and .delta.-amino valeric acid), acetylcholine (ACh), dopamine (DA) and l-Phe-Tyr inhibit an identifiable giant neuron (the TAN, tonically autoactive neuron) of A. fulica. These effects were examined by microdrop application in 2 different conditions: in physiological solution and in the absence of Cl-. The relatively transient (rapid) inhibitions caused by GABA, its derivatives and ACh are dependent on Cl-; the relatively maintained (long-lasting) inhibitions, caused by DA and l-Phe-Tyr, are independent of Cl-.