Transferrin accumulates within neurons of the developing nervous system of humans, sheep, pigs and chickens. To assess the relationship of this accumulation with the ontogeny of oxidative metabolism, we studied the immunocytochemical localization of transferrin (Tf) and the mitochondrial form of malate dehydrogenase (mMDH) in developing neural tissues by the peroxidase-antiperoxidase method. Rabbit anti-rat Tf was obtained commercially and gave a single band of reaction product (MW = 80 kd) on Western blots. Antibodies to porcine heart mMDH were elicited in a rabbit. Western blot analysis showed that this anti-porcine mMDH antibody reacted with the mMDH from porcine, rat or avian tissue but not with the cytosolic MDH from pigs. Tf was first detected in rat brain neurons at about the 18th embryonic day and reached a peak at about the 6th postnatal day. All neurons were immunoreactive with large neurons throughout the brain showing a strong reaction for Tf. From this time onward, the level in brain neurons gradually decreased until adulthood. However, Tf immunoreactivity still remained strongly evident in capillary endothelial cells. The localization of Tf within rat spinal cord neurons peaked as early as the 1st postnatal day and remained elevated to the 6th postnatal day. By contrast, reactivity for Tf within dorsal root ganglia neurons was intense as early as the 18th embryonic day and diminished only gradually. Mitochondrial MDH, a marker for oxidative metabolism, appeared to reach a peak after the crest of intraneuronal Tf had been observed. For example, brain and spinal cord MDH immunoreactivity increased with intense staining in the cell bodies and fibers of neurons from the 6th to the 13th postnatal day; immunoreactivity gradually diminished into adulthood. The gradient of reactivity was low in some areas of the brain but more intense in areas containing large neuronal cell bodies such as the red nucleus. This occurred after the peak of intraneuronal Tf at day 6 and suggested a precursor-product relationship. By contrast, immunoreactivity for neuron-specific enolase, a glycolytic enzyme, showed a developmental pattern that differed from either Tf or MDH in that reactivity appeared later in development and was less intense. These data suggest that as cerebral metabolic rates begin to increase as early as 5-6 days after birth in the rat, an increase in mMDH occurs coincident with the onset of oxidative metabolism. Furthermore, this rise in intraneuronal mMDH follows the peak of intraneuronal Tf and suggests that Tf supplies the iron required for the synthesis of other mitochondrial ferroproteins.