Many authors have stressed the role of serotonin in mood disorders and some aspects of schizophrenia. Cerebral serotonin synthesis depends on the quantity of circulating free tryptophan not bound to albumin. It thus appeared interesting to estimate the two forms of tryptophan by measuring their plasmatic concentration as well as the free/total tryptophan ratio. These evaluations were performed by an automatic spectrofluorimetric method. 19 blood samples were analyzed from normal subjects and 47 from schizophrenic or manic-depressive patients including treated and untreated cases. The only significant differences in the free/total tryptophan ratio were reported in untreated paranoid schizophrenics and in anxiety cases. A dual nosological and symptomatical approach was adopted in the course of attempted correlation with biological data in order to underline the possible role of symptomatic elements common to different diagnoses. The results for melancholia agreed with those reported in the literature (4). In the case of schizophrenia, however, there are only a few reports and no discrimination has been made between untreated and treated patients or between the different types of schizophrenia. The clinical picture is different for paranoid schizophrenia which is the delusive aspect of the disease and for hebephrenic schizophrenia. It seems fairly logical to find opposite results in delusive mania and paranoid schizophrenia compared to melancholia, since delusive mania and paranoid schizophrenia have similar symptomatology which is the opposite of that found in melancholic depression.