1. The respiratory rates in vitro of ten structures of the rabbit brain were measured in the presence of a normal (5mm) and an elevated (50mm) concentration of K+ ions. The results were expressed on a dry-weight basis and in terms of cell density. 2. On a dry-weight basis, with a normal concentration of K+, there was a steady decrease in respiratory rate on passing from the cerebral cortex through successively lower centres to the brain-stem, i.e. respiration was negatively correlated with phylogenetic age. 3. When the resting respiratory rates were expressed in terms of cell density there was no correlation with phylogenetic age. The neuron-containing structures of the cerebrum and brain-stem had identical respiratory rates with the exception of the cerebral cortex which had a higher rate. 4. The K+-stimulated respiratory rates/cell also showed a negative correlation with phylogenetic age; the higher centres responded with a greater percentage increase than the lower. 5. The results from the cerebellar cortex were anomalous. 6. There were variations of respiratory rate within the diencephalon. The thalamus had a relatively high respiratory rate, the anterior and ventral regions relatively low and the hypothalamus intermediate. 7. The oxygen consumption of white matter was not increased by a high external concentration of K+.

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