Plasma levels of several amino acids were studied in 14 patients with early stage probable Alzheimer's disease (AD) and 17 age-matched controls. In the AD patients a possible relationship between amino acid levels and behavioural symptomatology was also investigated. We found significantly reduced levels of tryptophan and methionine in plasma samples from the AD patients compared to the control subjects. Moreover, plasma tyrosine/large neutral amino acids (LNAA) ratio and the ratio of plasma taurine and the product of the plasma levels of methionine and serine (TSM-ratio) were significantly increased in the AD patients in comparison with the controls. However, no difference was found in plasma tryptophan/LNAA ratio and in homocysteine levels between both groups. Concerning the behavioural symptomatology no significant correlation was found between the Reisberg Behave AD scale and plasma amino acid levels or ratios. The reported findings suggest that abnormal amino acid metabolism is present in the early stages of AD. We hypothesize that this abnormality could play a role in the pathogenesis of behavioural changes occurring in later stages of AD.