Attempts to employ the numerous newly discovered psychotropic drugs to influence the course of mental disease have so far rested mainly on an empirical basis. These drugs cannot be said to "cure" mental disease, but, by their effect on the basic psychological processes, they can modify the personality structure and the perceptive, executive and integrative functions of the ego in such a way as to facilitate more adequate patterns of behaviour and strengthen the adaptive resources of the patient's personality. With the aim of developing a rational basis for the use of drugs in psychotherapy, the author examines the concepts of human personality developed by adopting a psychoanalytical approach and, in the light of these concepts, discusses the effects on behaviour observed following the administration of various psychotropic drugs. He then outlines the basic considerations that should be borne in mind when using these drugs for therapeutic purposes.