Immunocytochemical studies clearly showed that amyloid β-protein (Aβ) deposits are widely distributed in the subcortical regions as well as the cortices of normalaged and Alzheimer disease (AD) brains. To investigate the temporal profile of Aβ accumulation in the subcortical region, we quantitated Aβ40 and Aβ42 levels, using sensitive enzyme immunoassays, in the putamen and mammillary bodies of normal individuals aged 24 to 87 years and of AD patients. In these two regions, Aβ42 was the predominant species; in particular, Aβ42 was the only Aβ species detected in the putamen. In several cases the mammillary body contained only Aβ40, but not Aβ42. Although the extent of Aβ accumulation in the 2 subcortical regions was much less than that in the cortex of the same subject, Aβ42 appears to accumulate in both subcortical regions at the same time as in the cortex and leptomenings. In addition, the Aβ42 levels in the putamen or in the mammillary body correlated with those in the occipitotemporal cortex. This strongly suggests that the extent of Aβ42 accumulation in the brain is determined not only by the duration of Aβ accumulation but also by other unkown regional factors. Western blotting showed that the initial Aβ species to accumulate in the putamen or mammillary body varied among individuals. In some cases, an Aβ42 stable dimer was the most predominant species, while in other cases a 3 or 4 kD Aβ42 monomer was more abundant, suggesting that the clearance rates of the Aβ42 stable dimer and monomer are different in vivo.