The effects of nitric oxide (NO) and its synthase (NOS) inhibitor on the generation of spontaneous spreading depression (SD) waves in the frog retina were studied by recording spreading depression potentials (SDPs) as an index. NO-releasing reagents such as sodium nitroprusside and hydroxylamine as well as the substrate of NOS, L-arginine, increased the frequency of the occurrence of spontaneous SDPs. On the other hand, typical NOS inhibitors, Nω-nitro-L-arginine and Nω-nitro-L-arginine methyl ester decreased the frequency. These results imply the participation of NO and its releasing system in the generation of retinal SD. A possible model of SD-generation has been discussed from the viewpoint of potassium accumulation.