CO2 fixation by the nervous system has been demonstrated with the use of 14 C-labeled bicarbonate in the retina (Crane and Ball, 1951), in the brain of 1-d-old mice (Moldave et al., 1953), and in the adult cat brain (Berl et al., 1962a,b). When cat brain was perfused with labeled bicarbonate and the specific radioactivity of 14 CO2 was maintained at a constant level, most of the radioactivity fixed in the brain was found in free aspartate, glutamate, and glutamine, with a small amount in lactate (Berl et al., 1962a,b; Otsuki et al., 1963; Waelsch et al., 1964). Aspartate had the highest specific radioactivity, which approached about 10% of that of infused radioactive bicarbonate. When ammonia was administered together with H14 CO3 - to cats, the total amount of 14 CO2 -fixation