The conversion of U-14C-glucose and 1-14C-acetate was studied in rat brain tissue slices and human brain tissue. In both types of tissue, glucose was preferentially incorporated into the compartment with the large glutamate pool (probably localized in the neurones), while acetate was incorporated into the glutamate pool with rapid glutamine synthesis (probably in the glial cells). Glucose conversion to amino acids was 3.8 times greater in rat brain tissue than in human brain, but the utilization of acetate was only 1.34 times greater. These differences concur with the previously described lower neurone density and lower neuronal enzyme activities in man as compared with the rat and the almost equal glial cell density and glial enzyme activities in the two tissues.