AbstractThe effect of acetylcholine (ACh) on cone‐ and rod‐connected horizontal cells (generating photopic and scotopic L‐type S‐potentials, respectively) in the fish (Eugerres plumieri) retina was compared with that of dopamine (DA), and some similarities were found in the effects. About two‐thirds of the L‐type horizontal cells examined were sensitive to a test solution containing ACh (10 mM in concentration) and BW‐anticholinesterase (BW; 1.0 mM). ACh with BW augmented a center response and attenuated a surround response as did DA (0.2 mM), although the latter effect was more pronounced and longer‐lasting. ACh with BW frequently produced oscillations of the horizontal cell membrane potential as did DA with clonidine (α‐adrenergic stimulant). Phentolamine (α‐adrenergic blocker) interrupted both the effects of DA and ACh on the center and surround responses, while hexamethonium (cholinergic antagonist) appeared to interfere selectively with the effect of ACh. Therefore, the ACh‐induced changes observed in the horizontal cell membrane potential are assumed to be mediated by the adrenergic sy