Neuronal energy needs are mainly covered via mitochondrial oxidative phosphorylation. Even if the energy supply appears identical in immature and adult brain, nevertheless quantitative differences exist. The present study focuses on the adaptations in cellular energy metabolism caused by the neuronal maturation. As main parameters of oxidative phosphorylation, cellular oxygen consumption and mitochondrial membrane potential were measured in isolated rat cortical cells using a Clark-type oxygen electrode and microfluorometric techniques. In four age groups (E18–P2, P8–P12, P16–P20, ≥ P28), unstimulated neurons showed a significant age-dependent increase in basal oxygen consumption (6.1 up to 10.2 nm/min/107 cells). The excitatory neurotransmitter glutamate induced a further, but age- and concentration-independent, elevation of oxygen consumption to a plateau ≥ 14 nm/min/107 cells and a complete depolarization of mitochondrial membrane in neurons ≥ P8. Stimulation using K+ (5–50 mm) effected a concentration- and age-dependent increase in oxygen consumption, but a similar nearby complete depolarization of mitochondrial membrane in all tested age groups. Furthermore, uncoupling mitochondrial membrane function followed by a complete depolarization of mitochondrial membrane showed a maximal oxygen consumption (14–15 nm/min/107 cells) only in neurons ≥ P8. These data suggest that developing and adult cortical neurons cover their increased need of energy following stimulation by an efficiency improvement of mitochondrial oxidative phosphorylation. The age-independent limited capacity of mitochondrial oxidative phosphorylation, however, causes a reduction in cellular energy disposal in mature neurons and therefore may play a critical role in the increased sensitivity of adult neurons against excitotoxicity and ischaemia.