Neurofibrillary tangles in Alzheimer's disease show a predilection for cortical pyramidal and subcortical projection neurons. The antigenic composition, neuronal specificity and distribution of aluminum-induced neurofibrillary degeneration were examined in regions of rabbit brain analogous to those that develop neurofibrillary tangles in Alzheimer's disease. Neurofibrillary degeneration was induced by intraventricular instillation of aluminum chloride. In aluminum-treated rabbits, intensely immunoreactive filamentous aggregates were seen in affected neuronal perikarya after staining with an antiphosphorylated neurofilament antibody (SMI 31), while in controls immunoreactivity was confined to axon-like elements. Monoclonal antibodies against Microtubule-associated protein 2 and tau, which stain human neurofibrillary tangles, did not stain aluminum-induced neurofibrillary degeneration. Pyramidal neurons exhibiting neurofibrillary degeneration formed a discrete linear pattern in layers III and V of cortex. Cortical somatostatin and nicotinamide adenine dinucleotide phosphate diaphorase-reactive neurons identified in double-stained sections were unaffected. Large perikarya in the vicinity of the globus pallidus, some of which contained acetylcholinesterase, were frequently SMI 31-immunoreactive. Among the cell groups affected in the upper brainstem were the nucleus raphe dorsalis and locus coeruleus. These findings show that aluminum-induced neurofibrillary degeneration differs antigenically from neurofibrillary tangles in Alzheimer's disease. Nevertheless, many neuronal subsets that are particularly susceptible to Alzheimer's disease, including cortical pyramidal neurons, basal forebrain cholinergic neurons and upper brainstem catecholaminergic neurons, are also affected by aluminum-induced neurofibrillary degeneration.