Effects 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. gamma-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 beta-alanine (10 mM each) had negligible effects, although the former slightly depolarized and the latter merely hyperpolarized both cone- and rod-horizontal cells.