The total activities of lactate dehydrogenase (LD) and hydroxybutyrate dehydrogenase (HBD), and the LD isoenzyme distribution were determined in tissue samples taken in vivo from different parts of the rabbit brain, and in cortical grey, subcortical white, and cerebellar tissue from man. The total LD and HBD activities were found to be higher in the brain stem than in the cerebral hemisphere, basal ganglia, and cerebellum of the rabbit brain. The isoenzyme distribution in the brain stem and the cerebellum showed an anodal shift, i.e., significantly higher activities of LD1 compared with the cerebral hemisphere and the basal ganglia. In man the total LD activity was found to be lower in white than in grey matter, although the isoenzyme patterns did not differ. The implications of these findings in relation to the LD isoenzyme pattern in serum after traumatic brain injury are discussed.