Alzheimer's disease can be considered a late-onset system degeneration, characteristically involving certain populations of cholinergic neurons but eventually involving other cells as well. Decreases in cerebral metabolic rate occur in it and may reflect not only decreased neuronal activity, but also deficiencies in metabolic enzymes. Abnormalities reported in nonneural Alzheimer tissues suggest that at the molecular level it is a systemic disease whose biochemical aspects can usefully be studied in nonneural tissues. Alzheimer's disease can be formulated as one of a number of metabolic encephalopathies that impair central cholinergic function.