A micromethod for the polarographic measurement of oxygen consumption is used to study ADP-dependent respiration in mitochondria isolated from regions of the developing rat brain and liver. Mitochondrial preparations, from brains of rats 5–67 days of age, have very high respiratory control ratios, which do not change with the age of the animal. With the NAD-linked substrate pair, glutamate and malate, mitochondria from all brain regions show ADP/O ratios which increase from values of 1–2 in animals less than 2 weeks of age to the accepted mature value of 3 in animals older than 4 weeks. These differences are highly significant in all regions, but the rates of change vary in different regions. With succinate as the substrate, ADP/O ratios do not change significantly with the age of the animal.