In this study, calbindin D-28k (CaBP), monoamine oxidase A (MAO A) and nerve growth factor receptor (NGFr) immunoreactivities were investigated in the nucleus basalis of Meynert (NbM) in patients with senile dementia of the Alzheimer type (SDAT), with Parkinson's disease (PD) with or without dementia, and in controls. Immunocytochemistry using specific antibodies in differing serial sections was employed, and cell counts and NbM nuclear volume measurements were made. Most of the large multipolar NbM neurons showed CaBP immunoreactivity in the cytoplasm of their somata, dendrites and axons. In adjacent, NGFr-reacted sections, the large NbM neurons were also found to be intensely immunoreactive for NGFr on their cellular surfaces. In addition, a subpopulation of large NbM neurons and glial cells were found to be immunoreactive for MAO A. The number of CaBP-immunoreactive (CaBP-i) neurons was decreased by an average of 55% in the 6 SDAT patients, 70% in the 2 nondemented PD patients and 40% in the 1 demented PD patient. The volume calculated for the compact part of the NbM formed by the CaBP-i neuronal somata decreased by an average of 47% in SDAT. On the other hand, measurements in the volume of NGFr-i neurons (including the dendritic arborization) showed an average decrease of 25% in SDAT patients compared to controls. Although all SDAT and PD patients showed a decrease of CaBP-i neurons in the NbM, a loss of MAO-A-i NbM neurons was found only in those patients with dementia. Therefore, the relative proportions of MAO-A-i to CaBP-i neurons were increased in the nondemented PD patients (14.2 and 19.6%) when compared with those in the demented PD patient (2.2%) and with the SDAT patients (0.3-5.6%). These data indicate that a balanced presence of MAO-A-i cholinergic, large NbM neurons may be necessary for the proper maintenance of cognitive function. Functionally this may be translated to mean that dementing changes may cause a decrease from the normal amount of MAO A enzyme activity. This suggests that therapeutic strategies based upon correction of MAO-A activities by MAO-A inhibitors may be important to ameliorating some of the loss in cholinergic function in dementias of SDAT and PD.