We investigated whether the inhibition of nitric oxide (NO) synthesis with N ω-nitro- l-arginine methyl ester ( l-NAME), a competitive inhibitor of NO synthase, affects N-methyl- d-aspartate (NMDA)-induced neurotoxicity in the rat retina in vivo. A single intravitreal injection of NMDA damaged the ganglion cell layer and the inner plexiform layer without affecting the other retinal layers 7 days after injection. Intravitreal injection of (5 R,10 S)-(+)-5-methyl-10,11-dihydro-5 H-dibenzo[ a, d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) with NMDA significantly reduced NMDA-induced degeneration of the retina. NMDA-induced degeneration was also prevented by intravitreal injection of l-NAME but not of d-NAME. The protective effect of l-NAME was antagonized by l-arginine. These results suggest that NO plays an important role in NMDA-induced excitotoxic degeneration in the retina.