Distribution patterns of activity and immunohistochemical staining for glutamate dehydrogenase were compared during the postnatal development of rat hippocampal formation and cerebellar cortex. On postnatal day 5, dendritic layers of the hippocampal formation showed only a very weak enzyme activity. Similarly, when studied at the same age, the external granule cell layer and Purkinje cells of the cerebellar cortex exhibited a very faint and moderate staining, respectively. With advancing age, in both brain regions a marked postnatal increase in glutamate dehydrogenase activity occurred in neuropil area as glutamatergic structures matured. However, compared to activity staining, both brain regions of early postnatal stages showed a relatively high level of glutamate dehydrogenase-like immunoreactivity. In this case, the immunohistochemical staining of hippocampal dendritic layers and of the molecular layer of the cerebellar cortex was rather diffuse, being not very similar to parameters of the maturation of the respective glutamatergic structures. In contrast to the activity staining for the enzyme, the immunohistochemical labelling in adult rats revealed a selective predominance of immunoreactivity in astroglial cells from postnatal day 5 onwards. The Bergmann glia in the cerebellar cortex exhibited the strongest intensity of immunoreactivity. Generally, the patterns of immunoreactivity were found to depend on the fixation procedure adopted. Concluding from our results, glutamate dehydrogenase is demonstrable in glial and in neuronal cell elements as well. Therefore, it is recommended that activity staining and the immunohistochemical procedure be combined to study qualitative and quantitative aspects of glutamate dehydrogenase in nervous tissues.