Functions of nitric oxide are of common interest among a variety of tissues, since it activates soluble guanylate cyclase to produce cGMP. Here we report that intracellular application of L-arginine, the precursor of nitric oxide, blocked gap junctions between horizontal cells of the turtle retina. The input resistances of the cells were greatly increased and the cells were thereby easily polarized by current injections through microelectrodes. This procedure enables us to plot precise I-V curves and the reversal potential of light responses was estimated at around 0 mV. These results were quite similar to those obtained by intracellular application of cGMP, suggesting that the L-arginine:nitric oxide:cGMP pathway is present in retinal horizontal cells.