Effects of the following amino acids were examined on the electrical activity of the two giant neurones (PON and TAN) identified in the subesophageal ganglia of Achatina fulica Férussac : L-Asp, L-Thr, L-Ser, L-Glu, L-Pro, Gly, L-alpha-Ala, beta-Ala, L-cysteine, L-cystine, L-Val, L-Met, L-Ileu, L-Leu, L-Tyr, L-Phe, L-Lys, L-His, L-Arg, L-Cit, L-Try, GABA and GABOB. Among these substances, we observed an inhibitory effect of GABA and GABOB on the TAN excitability. GABA showed stronger effect on the TAN than GABOB. This effect of GABA was due to producing hyperpolarization on the TAN membrane. GABA showed a slight excitatory effect on the PON. The effect of GABOB on the PON was very weak and unstable.