This essay reconstructs a social and cultural history of "truth serum" in America during the 1920s and 1930s, identifying the intellectual ingredients of the idea of a physiological "truth technique," and examining why it seemed to meet an urgent need. It argues that truth serum had the patina of modern science but produced a phenomenon that could be understood and evaluated by everyman. It therefore offered the public a technique with the benefits of expertise but without its attendant costs to lay authority. The paper also argues that truth serum helped develop an account of memory as a permanent record of experience, accessible through altered states of mind. This view contributed to the production of a public understanding of memory that both diverged from previous claims about memory and recall, and ran counter to the direction of current psychological research. It thus helped lay the groundwork for claims about memory permanence and scientific recall techniques later in the twentieth century.