Photopic L-type responses were recorded from the soma and the axon terminal of horizontal ceIls of the catfish (Zctalu~us punctutus) retina in eye-cup preparations. The responses were produced by a spot of light with 100-pm diameter (intensity, 10 pW1 cm2), which was flashed or steadily illuminated and swept along a 6-mm length over the retinal surface at a speed of 0.95 mmlsec. In some experiments, a traveling random bar stimulus was used instead of the sweeping spot. While recording the responses, dopamine (DA) was applied in a jet form via a nebulizer over the retinal preparation or as superfusate to the eye-cup preparation. DA increased the response amplitude by about 50% and markedly narrowed the spatial profile of the responses from the soma but not from the axon terminal. These DA effects were observed in both normal retinas and those from which DA cells had been deprived by prior intraocular injection of 6-hydroxydopamine. Deprival of DA cells from the retina resulted in a slightly wider spatial profile of the soma and axon terminal responses than that in normal retinas. The results indicate that the spatial properties of photopic L-type responses are modulated by DA at the soma level of horizontal cells.