ABSTRACT Several lines of evidence suggest that nitric oxide (NO) can regulate diverse retinal functions, but whether this gas is capable of modulating the visual responses of retinal output neurons has not been established. In the present study, the effects of NO on rod-driven responses of retinal ganglion cells were tested by making whole-cell patch-clamp recordings from morphologically identified ganglion cells in the isolated ferret retina. Bath application of L-arginine, the substrate of nitric oxide synthase, and SNAP, the NO donor, was found to differentially affect On and Off discharge patterns. The introduction of these drugs significantly decreased visual responses of retinal ganglion cells, but the effects were more pronounced on Off than on On discharges. The peak discharge rates of On responses were usually reduced by about 40%, but not completely blocked. In contrast, Off responses were completely blocked in most cells. These differential effects were observed in On-Off cells as well as in cells that yielded just On or Off discharges. The Off responses that were blocked by NO were also blocked by APB and strychnine, suggesting the involvement of the APB-sensitive rod pathway.