It has been recognized for some time that it can be important for the institution of rational pharmacotherapy to know the blood concentration of the agent used (Brodie (1964)). I n pharmacology, however, studies of the relation between blood concentration and therapeutic effect are scarce. There are indications of the existence of such a relation in the field of antidepressants (Hammer et al. (1966), Asberg et al. (1971, Braithwaite et al. (1972)).This paper reports on a study of the relation between the plasma concentration of thioridazine and its therapeutic effect as an antidepressant. This phenothiazine derivate is known to have not only a neuroleptic but also an antidepressant effect, a fact which has been pointed out since its introduction (Cohen (1958)), and which is emphasized also in recent literature