The regional distribution and cellular localization of aspartate aminotransferase (AspAT) as a glutamate and aspartate metabolizing enzyme was studied in the hippocampal formation, in dorsal root ganglia, and in superior cervical ganglia during the postnatal development of the rat. At birth, in all neuronal tissues studied the enzyme activity was rather low. Whereas in the dendritic layers of the hippocampal formation enzyme levels rose strikingly during the first weeks of postnatal life, those of peripheral ganglia remained remarkably stable. The high affinity uptake of [3H]labelled L-glutamate and D-aspartate were studied in surface autoradiograms of incubated slices of hippocampal formation during postnatal development of the rat. Our data indicate that the postnatal development of high affinity uptake capacity is parallel to the increase of histochemically demonstrable AspAT activity in the hippocampal formation suggesting that the enzyme may be involved in glutamate/aspartate neurotransmitter metabolism.