PURPOSE. To evaluate the potential neuroprotective effects of C-type natriuretic peptide (CNP) on rat retinal ganglion cells (RGCs). METHODS. Cultured adult rat retinal cells were treated with vehicle, CNP, or atrial natriuretic peptide (ANP), followed by cytotoxic insults (glutamate, TNF alpha, or withdrawal of trophic factor). RGC survival was analyzed by counting Thy-1-positive cells in each well. For in vivo evaluation, N-methyl-D-aspartate (NMDA) with or without CNP was injected intravitreally into rat eyes. At various time points after injection, retinal cross-sections were analyzed for thickness changes in the retinal layers, and retinal flat mounts were assessed by counting cresyl violet-labeled or TUNEL-positive cells. Expressions of natriuretic peptide receptor-B (NPRB) and apoptosis-related genes in retina, including Bcl-xL, BAX, and mu-calpain, were analyzed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). RESULTS. At 50 and 500 nM, CNP, but not ANP, significantly (P < 0.05) protected against glutamate-insult and trophic factor withdrawal-induced RGC death in vitro. Neither peptide significantly affected TNF alpha-induced cytotoxicity. Intravitreal injection of NMDA (20 nanomoles) significantly (P < 0.05) decreased the thickness of the inner plexiform layer (IPL), induced cell loss, increased the number of TUNEL-positive cells in the RGC layer, and upregulated the expression of Bcl-xL, BAX, and mu-calpain. All these effects were significantly (P < 0.05) alleviated by concomitant injection of CNP (4.5 nmol, 10 mu g). The neuroprotective effects of CNP were maintained up to 14 days after CNP injection. CONCLUSIONS. CNP protects rat RGCs against the apoptotic damage induced by insults such as excitatory amino acid, both in vitro and in vivo. (Invest Ophthalmol Vis Sci. 2010;51:3544-3553) DOI: 10.1167/iovs.09-5049