Monocarboxylate Transporter3 (MCT3), encoded by Slc16a8, is a proton-coupled lactate transporter localized to the basolateral membrane of the retinal pigment epithelium (RPE), where it facilitates lactate and proton efflux to the choroid. Its expression is highly tissue-specific, mainly restricted to the RPE and choroid plexus epithelium. (1,2) MCT3-deficient mice (Mct3⁻/⁻) exhibit reduced scotopic electroretinogram (ERG) amplitudes despite intact retinal morphology and healthy photoreceptors, suggesting impaired visual function arises from subretinal acidosis due to lactate accumulation. (3) Loss of basolateral CD147, essential for MCT3 localization, further disrupts this transport system. (3) MCT3, together with apically localized MCT1, enables directional lactate clearance from the retina, underscoring its role in maintaining subretinal pH and metabolic balance. (2) These insights position MCT3 as a key regulator of the retinal microenvironment.