AbstractEffects of some amino acids on horizontal cells were examined in the fish (Eugerres plumieri) retina. L‐aspartate (5 mM in concentration) and L‐glutamate (10 mM) consistently depolarized all types of horizontal cells, although the effect of L‐aspartate was more pronounced. γ‐Aminobutyric acid (GABA; 10 mM) consistently hyperpolarized the cone‐connected L‐ and C‐type horizontal cells, whereas the effect on the rod‐connected cells was slight or nonexistent. The hyperpolarizing effect of GABA on the cone‐horizontal cells was markedly diminished by picrotoxin (0.1 mM). Usually, glycine (10 mM) showed slight and variable effects. It merely depolarized some cone‐horizontal cells but weakly hyperpolarized some rod‐horizontal cells. In a few cases, however, glycine began to drastically hyperpolarize the cone‐horizontal cells in retinas that had been repeatedly treated with certain agents used. Taurine and β‐alanine (10 mM each) had negligible effects, although the former slightly depolarized and the latter merely hyperpolarized both con