1. The cytochrome-oxidase activity in eleven structures of the rabbit brain has been investigated. 2. Kinetic data suggest that the same enzyme occurs in all brain structures but that the activity on a tissue-weight basis differs from structure to structure, being in general higher in the phylogenetically newer structures. 3. When the kinetic data are related to cell density (as measured by DNA concentration) all the neuron-containing structures of the cerebrum and brain stem show the same activity but the corpus callosum and the cerebellum show much less. 4. A special study was made of the distribution of activity and cell density within the diencephalon. 5. The results suggest that all neurons in the cerebrum and brain stem have a similar cytochrome-oxidase activity, which is about 80 times that of glia, and that the cerebellum is relatively richer in glia or that it contains neurons with a much lower cytochrome-oxidase activity, or both.