We analyzed the amino acid concentrations and aminopeptidase activities in plasma and the cerebrospinal fluid (CSF) of Alzheimer patients. Correlations were calculated between these parameters and the degree of cognitive impairment, duration of the disease and age. In comparison with a control group, no changes were found in the levels of amino acids in CSF or plasma of Alzheimer patients. Alanyl-aminopeptidase activity was significantly lower in the CSF of Alzheimer patients, whereas no differences in CSF were detected as regards the remaining aminopeptidases. The plasma/CSF ratio for aminopeptidase activities was higher in AD patients than in controls, although the difference was only significant for alanyl-aminopeptidase. Amino acid ratios did not show this general tendency. The correlations between plasma aspartate and glutamate concentrations and the stage of the disease, measured with the Mini Mental State Examination, were studied. Changes in aminopeptidase activities and their role in protein dysfunction underlying Alzheimer disease are discussed.