The use of organ extracts to treat psychiatric disorder in the interwar period is an episode in the history of psychiatry which has largely been forgotten. An analysis of case-notes from The Maudsley Hospital from the period 1923–1938 shows that the prescription of extracts taken from animal testes, ovaries, thyroids, and other organs was widespread within this London Hospital. This article explores the way in which Maudsley doctors justified these treatments by tying together psychological theories of the unconscious with experimental data drawn from laboratory studies of human organs. It explores the logic behind these treatments and examines beliefs about their efficacy. The connection between this historical episode and current research in endocrinology and psychology is explored.