Tryptophan has attracted attention during recent years for the possible role played by some of its metabolites in the aetiology and treatment of psychiatric disorders. Of these metabolites, serotonin has received special attention and considerable work has been carried out in order to elucidate its physiological and pharmacological actions. Studies from animal experiments are thus available on the function of serotonin in nervous activity (Marrazzi and Hart, 1955; Brodieet al., 1955; Pletscheret al., 1956; Welsh, 1957), but the results obtained thus far in psychiatric patients are uncertain. Recent researches in the affective psychoses (Burgermeisteret al., 1963) did not reveal significant differences between patients and normal controls in the urinary output of 5-hydroxyindoleacetic acid (5-HIAA) before or after an oral 5-hydroxytryptophan load.