The density and distribution of senile plaques and neurofibrillary tangles were examined in the olfactory bulbs and tracts, amygdala and hippocampus of 28 patients with Alzheimer's disease, 13 with Down's syndrome and 60 non-demented patients of age range 6-84 years. In all three patient groups comparisons of incidence and severity over the three areas showed the amygdala to be the most commonly and most severely affected area by senile plaques, the hippocampus by neurofibrillary tangles, and the olfactory bulbs and tracts to be the least affected by both. These findings are discussed in relationship to the possibility that the olfactory tracts might provide a portal of entry to the brain for any putative pathogenic agent(s) that might be responsible for the induction of senile plaques and/or neurofibrillary tangles.