Some clinical reports indicate that exogenous CDP-choline (citicoline) may have a therapeutic effect in patients with glaucoma. However, the precise effect of citicoline on damaged retinal ganglion cells (RGCs) remains to be explained. We performed tissue culture of mouse retinal explants and investigated the effect of citicoline on damaged RGCs by the quantitative analysis of TdT-dUTP terminal nick-end labeling (TUNEL) staining and the assessment of the number of regenerating neurites. The TUNEL-positive ratio in 0.1-10 micromol/l citicoline-treated retina was very low, and the number of regenerating neurites increased more than in control retina. Our findings suggest that citicoline has a protective effect on damaged RGCs in tissue culture of retina.