The possible role of nitric oxide (*NO) in brain mitochondrial maturation was studied. Within the first 5 min after birth, a sharp increase in ATP concentrations was observed, coinciding with an increase in mitochondrial complex II-III (succinate-cytochrome c reductase) activity, while complex I (NADH-CoQ1 reductase) and complex IV (cytochrome c oxidase) activities remained unchanged. Under the same circumstances, cGMP concentrations were increased by 5 min after birth, correlating significantly with ATP concentrations. Since ATP concentrations also correlated significantly with mitochondrial complex II-III activity, these three parameters may be associated. Inhibition of *NO synthase activity brought about by the administration of N(omega)-nitro-L-arginine monomethyl ester to mothers prevented the postnatal increase in cGMP and ATP levels and complex II-III activity. These results suggest that early postnatal mitochondrial maturation in the brain is a *NO-mediated process.