Although previous Northern blot and in situ hybridization studies suggested that neurons express the monocarboxylate transporter MCT2, subsequent immunohistochemical analyzes either failed to confirm the presence of this transporter or revealed only a low density of immunolabeled neuronal processes in vivo. The authors report that appropriate section pretreatment (brief warming episode or proteinase K exposure) leads to extensive labeling of the neuropil, which appears as tiny puncta throughout the whole mouse brain. In addition, intense MCT2 immunoreactivity was found in cerebellar Purkinje cell bodies and their processes, on mossy fibers in the cerebellum, and on sensory fibers in the brainstem. Double immunofluorescent labeling with appropriate markers and observation with epifluorescence and confocal microscopy did not show extensive colocalization of MCT2 immunoreactivity with presynaptic or postsynaptic elements, but colocalization could be observed occasionally in the cortex with the postsynaptic density protein PSD95. Observations made at the electron microscopic level in the cortex corroborated these results and showed that MCT2 immunoreactivity was associated with wide membrane segments of neuronal processes. These data provide convincing evidence that MCT2 represents a major neuronal monocarboxylate transporter in the adult mouse brain, and further suggest that mature neurons could use monocarboxylates such as lactate as additional energy substrates.