We evaluated to what extent the tyrosine (TYR) and tryptophan (TRP) transport by the red cells was stable in normal subjects during divers clinical conditions. Twenty-two normal subjects were studied. The values of TYR and TRP transport were found to be individually determined, showed no circadian oscillations and no changes during the menstrual cycle, but varied somewhat more during the postpartum period. A few normal subjects had low TYR and TRP transport values, as observed in patients with affective disorders.