The effects of nitric oxide (NO) on electroretinograms and light responses of horizontal cells intracellularly recorded from isolated, superfused carp retinas were studied. Sodium nitroprusside (SNP), an NO donor, suppressed scotopic b wave, while enhancing photopic b wave, and the effects could be blocked by hemoglobin, an NO chelator. Furthermore, following SNP application, light responses of rod horizontal cells were reduced in size and those of cone horizontal cells were increased. These results suggest that NO suppresses the activity of rod pathway, but enhances that of cone pathway in the outer retina. Moreover, the effects of methylene blue, an inhibitor of soluble guanylate cyclase, on rod and cone horizontal cells were just opposite to those of SNP, implying that the effects of NO may be mediated by cGMP.