ABSTRACT. Plasma concentration of tryptophan, tyrosine, leucine and thiamine were reduced in senile dementia of Alzheimer type. In Down's syndrome, where Alzheimer‐type histology appears consistently at an early age, there was a definite type of abnormality, raised concentrations of isoleucine, leucine, phenylalanine and cystine. Because of the competition between amino acids for transfer into the brain, either or both of these types of change could lie in the aetiological chain underlying the development of Alzheimer pathology. Alternatively, these patterns could reflect the requirements of aberrant central/peripheral protein turnover.

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