Experiments in which we investigated the possible oxidative utilization of lipoid substrates by brain and liver mitochondria were carried out with rats aged 5 and 90 days, kept under completely standardized conditions. Brain mitochondria were isolated on a Ficoll gradient after Clark and Nicklas (1970). Respiratory activity (or the respiratory control index-R.C.) was determined in the manner described in an earlier paper (Dobesová and Mourek 1980). Na succinate or Na malate was used as the testing substrate; palmityl carnitine, acetyl carnitine and acetoacetate were used as lipoid substrates. Oxygen consumption was measured with a Clark's oxygen electrode and respiration was expressed in nAt oxygen per min per mg protein, which was measured by the method of Lowry et al. (1951). When using succinate or malate, in agreement with our previous results we did not find any development changes in the respiratory activity of the brain mitochondria. The oxidation of acetoacetate by the brain mitochondria of 5-day-old rats was about five times greater, and of acetyl carnitine over two times greater, compared with the CNS mitochondria of adult rats. The oxidative utilization of lipoid substrates by the liver mitochondria of 5-day-old rats was significantly greater than their utilization by CNS mitochondria (in the case of palmityl carnitine three times greater, for example) and was always significantly greater than in the liver mitochondria of adult rats. We demonstrated that mitochondria isolated from the brain of 5-day-old rats are equipped with an enzymatic apparatus which allows them to utilize lipoid substrates on a significantly greater scale than in adulthood.