In the cerebellar cortex of the rat, histochemically demonstrable activity of glutamate dehydrogenase (GDH) was found to increase markedly after the second week of postnatal life. As evaluated histophotometrically, the amount of reaction product in Purkinje cell perikarya exceeded on day 40 that on day 5 by about 85%, whereas the intensity of GDH staining in granule cell bodies and in the molecular layer rose from postnatal day 5 to day 40 up to 325% and 400%, respectively. Due to the parallelism with the onset of aminoacidergic transmission processes, the results are interpreted as indicating the participation of GDH in the metabolism both of transmitter glutamate and of gamma-aminobutyric acid (GABA).