The extent of CO2 fixation in the sciatic nerve of the rabbit was determined. The specific activitiy of citric acid was higher than that of glutamic, aspartic, and malic acids, and the specific activity of citric acid obtained from the 2 hour incubation nerve was close to 1/3 of that of the CO2 in the medium. The ratio of the radioactivity of the C-6 to C-1 of citrate was about 2∶1 in intact nerves and about 1∶1 in damaged nerves, and the ratio of the radioactivitiy of C-4 to C-1 of aspartate was approximately 1∶1 in both cases. These results suggest that in the sciatic nerve of the rabbit: 1) the dicarboxylic acid “shuttle” was active, 2) the extent of the carboxylation at the oxalosuccinic acid level was 1/2 or more of that at the oxaloacetic acid level, and 3) the CO2 fixation by the carboxylation of a-ketoglutaric acid might have some relationship to nerve function. The significance of CO2 fixation, and the possible relationship between the carboxylation of α-ketoglutaric acid and the concentrations of citric acid, acetyl-CoA and acetylcholine, and the control of the rate of tricarboxylic acid cycle were discussed.