The distribution of MAO A and B immunoreactivity was studied in the postmortem hippocampus, cerebellum and temporal cortex of normal controls and in patients with senile dementia of the Alzheimer type (SDAT). In normal control, neurons, glia cells, cellular processes and the walls of blood vessels were found to be MAO-immunoreactive (MAO-i). In the hippocampus of control cases, a subpopulation of small nonpyramidal neurons was MAO A-i. In addition, the somata of granule and pyramidal cells were densely surrounded by MAO B-i cellular processes. In the cerebellum, fine MAO A and B-i cellular processes and small varicosities were found in the cortex and in the dentate nucleus. In the normal temporal cortex, glia cells and cellular processes were MAO A-i and MAO B-i in all lamina. In SDAT, some changes of MAO immunoreactivity were encountered in all of the examined brain areas. The hippocampal MAO A-i neurons were well preserved in the dentate gyrus. However, large MAO B-i glia cells were found in the Ammon's horn, subiculum and entorhinal cortex, where they formed cell clusters or patches with increased MAO B immunoreactivity. Bodian-stained sections of the same hippocampi showed neuritic plaques in a similar number and regional distribution as the patches of increased MAO B immunoreactivity. In the cerebellar cortex of SDAT patients, plexuses of thick MAO A-i processes as well as patches of increased MAO B immunoreactivity with clusters of large MAO B-i glia cells were found. In the temporal cortex of SDAT cases, proliferation of large MAO B-i glia cells was detected. These glia cells formed numerous patches of increased MAO B immunoreactivity which were predominantly found in the upper cortical layers, similar to the distribution of neuritic plaques.