The study dealt with the level of and diurnal alterations in the concentration of tryptophan, free tryptophan and tyrosine in the blood plasma of 20 inhibited depression patients and 10 healthy controls. The results suggested that there was no distinct relationship between either the total plasma tryptophan or plasma tyrosine level and depression. On the other hand, the free plasma tryptophan level was, at all the times of day at which measurements were made, either significantly or almost significantly higher in the patients than in the controls. It was further found that the results of measurement were related to the patients' clinical improvement, as measured by the Hamilton test, in such a way that after four weeks of treatment the free plasma tryptophan level in 'poorly improved' patients continued to be significantly higher in comparison with the controls, whereas the values for the 'well improved' patient group did not differ greatly from the corresponding values for the control group any longer. It may be hypothesized that the rise in the free plasma tryptophan in depressive patients might represent an effort made by the peripheral body to compensate for the slowed-up serotonin metabolism of the brain, whereby the tryptophan mobilized from the periphery would serve as a sort of 'endogenous antidepressant' provided by the organism itself.