On the basis of the hypothesis of the brain monoamine metabolism abnormality in manic-depressive illness, plasma tryptophan (TRP) ratio, an indicator of brain 5HT level, and the ratios of other neutral amino acids (tyrosine (TYR), valine, isoleucine, leucine, and phenylalanine) were determined in manic and depressed patients. Our results suggest that, in manic-depressive illness, there might be abnormalities in the metabolism of neutral amino acids, mainly of TRP and TYR, and that abnormality of TRP might be related to the depressive symptoms, and that plasma TRP and TYR ratios might be important indicators for determining the efficacy of some anti-manic and anti-depressive drugs.