There is an age-dependent nerve cell loss in some areas of the brain, while other brain regions are stable with aging. The nucleus basalis of Meynert (NbM) is believed to be the source of cholinergic innervation of the cerebral cortex, and loss of its neurons seems to be followed by cognitive deficits. The normal age kinetics of the NbM are, therefore, of considerable importance. Sixteen autoptic human brains were examined, ages ranging from 35th week of gestation to 90 years of age. Blocks containing the NbM in its entirety were cut into 20-mu thick serial sections; every 25th section was cresyl-violet stained and underwent morphometric analysis. Nerve cell counts were slightly but significantly higher in the right hemisphere. The total number of neurons in the 9th decade was 23% below that in newborns. This decrease was statistically significant. We hypothesize that there is a threshold number of nerve cells below which cognitive failure is highly probable.