Recent work by the author and his colleagues (Sandison, Spencer and Whitelaw, 1954) has established that lysergic acid diethylamide is of great value in the psychotherapy of the neuroses. This paper attempts to examine in rather more detail the possible mechanism of action of the drug in terms of dynamic psychology. It is now generally accepted that the psychoneuroses are the result of a faulty relationship between the conscious and the unconscious, leading to a one-sided or prejudiced conscious point of view. Any discussion which follows this observation must be preceded by a definition of the writer's conception of the unconscious, and in this paper the standpoint adopted by Jungian analytical psychology will be preferred.