This study aims to promote long-term retinal ganglion cell (RGC) survival in a spontaneous glaucoma model by injecting slow-release Poly(DL-lactide-co-glycolide) (PLGA) microspheres containing glial cell line-derived neurotrophic factor (GDNF) into the vitreous. Microspheres (1 microL) suspended in PBS were injected in ipsilateral eyes while contralateral eyes served as untreated controls. Mice were injected at 2 months intervals (1-4 injections) depending on the protocol. ELISA assay indicated a cumulative GDNF release of 35.4 ng/mg over 71 days. The release was nonlinear with an initial burst of over 50%. Mice displayed a 30% drop in RGC density by 8 months (p = 0.013) and 80% drop by 10 months (p < 0.01). GDNF delivery increased RGC survival in all groups. Mice receiving early treatment showed up to 3.5 times greater RGC density than untreated mice at 15 months survival (p < 0.05). No significant effect was found in sham or lens injury groups. Microsphere-delivered GDNF significantly increases long-term RGC survival in a spontaneous glaucoma model, although the nonlinear release kinetics suggest that burst release may play a role in this rescue. Neuroprotection with slow-release polymers with improved release kinetics should be further studied as a potential therapy for glaucoma and other diseases involving the loss of central nervous system neurons.