Monocarboxylate transporters (MCTs) are a family of highly homologous membrane proteins that mediate the 1:1 transport of a proton and a lactate ion. In chicken, MCT3 is preferentially expressed in the retinal pigment epithelium (RPE). We have isolated the mouse MCT3 cDNA and gene and characterized the pattern of tissue expression. MCT3 is a single copy gene with a 1.8-kb transcript that encodes a protein with a predicted molecular mass of 51.5 kDa. Based on Northern hybridization analysis, MCT3 transcript was expressed in only two tissues: RPE and choroid plexus epithelium (CPE). The choroid plexus forms a barrier between the cerebrospinal fluid and fenestrated capillaries, similar to the organization of the RPE and choroidal vessels. Immunohistochemical staining demonstrated that MCT3 was restricted to the basolateral membranes of both epithelia but was more abundant in RPE than CPE. Differences in the level of protein expression were confirmed by Western blot analysis. The cloning of MCT3 identifies a specific transporter that could regulate lactate levels in fluid-bathing neuronal tissues.