Within the reticulate core of the brain the nucleus raphe dorsalis (NRD) is a major site of predilection for the neurofibrillary tangle (NFT) in dementia of the Alzheimer type (DAT) and, according to some studies, its primary brain stem site. In a quantitative study we have shown in six aged control brains an average of 5.2 NFT per histological section or 25.8 per mm3 and in seven age-matched brains with DAT a highly significant six-fold increase, respectively 35.3 NFT per histological section or 188.5 per mm3. There was no significant difference between the two groups in overall neuronal cell packing density. There was, however, a significant decrease in a subpopulation of neurons in DAT, a large polygonal neuron. Despite the prominence of the NRD as a target for NFT, the actual proportion of affected NRD cells was small, 0.35% in controls and 2.25% in DAT. © 1984 by the American Association of Neuropathologists.