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Neurones are dependent on their mitochondria to produce the necessary amounts of ATP for survival. Retinal ganglion cells (RGCs) have a particularly large number of mitochondria which—unlike neurones in the brain—are exposed to visual light of 400–850nm. Here we demonstrate that short wavelength visual blue light negatively affects mitochondrial function, causing oxidative stress and decreased cell survival. In contrast, long wavelength red light enhances mitochondrial function to increase survival of cultured R28cells and reduce the effects of blue light. Induction of retinal ischemia for 60min in dark conditions caused a reduction in ATP levels accompanied by decreased RGC numbers in all areas of the retina. These effects were diminished when ischemia was induced with concomitant delivery of red light, and exacerbated when blue light was used. We conclude that while the levels of blue light that reach the human retina will be a fraction of those used in the present study, the chronic nature might, on a theoretical basis, be detrimental to RGC mitochondria which are already affected by conditions such as glaucoma. Our findings also show that exposing the retina to red light may be a therapeutic approach to supporting healthy mitochondrial functions as part of the treatment for retinal diseases in which these organelles are affected.Blue and red light differentially affect mitochondrial functions.R28cells in culture are affected negatively by blue light or mitochondrial toxins and are reduced by red light.Retinal ganglion cell death caused by ischemia is exacerbated by blue light but attenuated by red light.Increasing levels of red light on the retina might be used to treat glaucoma where mitochondrial dysfunctions occur.