AbstractThe effects of catecholamines (CA) and certain related compounds in the superfusate were examined on the intracellularly recorded potential from horizontal cells in the fish (Eugerres plumieri) retina. The stimulated retinal area consisted of a central spot 1.0 mm in diameter and an annulus 2.0 mm in inner diameter and 4.0 mm in outer diameter; both forms of monochromatic stimuli were centered relative to the recording microelectrode. Each of the CA (dopamine, noradrenaline, and adrenaline) produced an analogous effect on the hyperpolarizing response of all types of horizontal cells. The depolarizing response of the C (R/G)‐type cells was found to change variably with the CA. The effect of dopamine (DA) among the CA was most pronounced when they were used at an equivalent amount. With 10–50 μM, the action of DA was variable but in general its effect was to increase slightly both center and surround responses. In some cases, however, DA initially augmented the surround and reduced the center response. Large amounts of the CA (100–200 μM) augmented the center response and attenuated the surround response considerably; these reciprocal changes usually were associated with moderate depolarization of the cells (5–10 mV). Recovery then occurred in 15–20 minutes. These results indicate that the CA, up to a certain amount, do not directly affect the transmission from photoreceptors to horizontal cells, since the center response became larger. At the same time, the lateral propagation of an S‐potential appears to be selectively affected by the CA, suggesting that the adrenergic system participates in this phenomenon. When an excess of these compounds (200–500 μM) was given, the cells were rapidly depolarized to near 0 mV and eventually the light‐induced responses were abolished. Large amounts (5–10 mM) of metabolic products of the CA (DOPAC and VMA) were found to reduce the center response slightly more than the surround. α‐Methylnoradrenaline, 5‐hydroxydopamine, and serotonin also caused the same but less effect on horizontal cells as did DA. Reserpine and clonidine mimcked the CA effect only if these compounds had been preceded by repeated applications of one of the CA or if the retina had been pretreated with Marplan®. Propranolol, haloperidol, and apomorphine affected neither the horizontal cell membrane potential nor the CA effect. However, phentola‐mine in large amounts (500 μM) markedly diminished the DA action. Therefore, α‐adrenergic receptors appear to he involved in the UA‐induced changes observ