Excitotoxicity is thought to be involved in the neuronal cell death induced by glaucoma. It has been reported that cannabinoid CB₂ receptor stimulation protects against light-induced retinal injury and tunicamycin-induce photoreceptor cell injury. In the present study, we examined whether PM226, a cannabinoid CB₂ receptor agonist, protected retinal ganglion cells against the NMDA-induced neurotoxicity in the mice, in vivo. Male ICR mice of 8-12 weeks old were subjected to intravitreal NMDA (40 nmol/eye). PM226 (10~100 pmol/eye) was intravitreally injected simultaneously with NMDA. Eyes were enucleated 7 days after NMDA injection, and the whole mount retinas were prepared. Immunohistochemistry using Alexa Fluor 488-conjugated anti-NeuN antibody was carried out to visualize retinal ganglion cells in the whole mount retina. PM226 (100 pmol/eye) significantly reduced retinal ganglion cell loss 7 days after NMDA injection. These results suggest that PM226 protects retinal ganglion cells against excitotoxicity, and that cannabinoid CB₂ receptor stimulation may be protective against excitotoxicity in the mice retina.