Mapping of striatal and diencephalic plaque distribution was conducted in 25 cases of dementia of the Alzheimer type. This analysis was carried out by fluorescence microscopy of paraffin-embedded tissue sections treated with Thioflavine S as fluorochrome. Consistent differences in plaque morphology and density between nuclei and fiber tracts were observed. Striatal and pallidal distribution was uneven, with plaque aggregation near and within certain fiber tracts: capsules, medullary laminae, and radial fasciculi. Diencephalic plaques showed also preferred aggregation near and within fiber tracts and within the intralaminar nuclei. The different subcortical plaque morphologies observed according to the nuclear or fiber tract location of the amyloid plaque, indicates that the peripheral ("halo") portion of the plaque is determined by the neurophil response to the primary event: the amyloid deposit. No correlation was observed between the distribution of plaques and any particular neurotransmitter system. In that respect, plaques were present within the nucleus basalis. Neurofibrillary tangle distribution was also seen to be dissociated from plaque distribution.