The Wellcome Trust’s Research Resources in Medical History has funded a one-year-long project to catalogue the Royal Society’s twentieth-century medical collections. As part of this project, the papers of British pharmacologist Sir John Henry Gaddum FRS have been catalogued. The collection spans Gaddum’s entire career, from school writings and first forays into science to his final post as Director of the Institute of Animal Physiology, Babraham, Cambridge, in 1958. In addition to a wealth of working papers, laboratory notebooks and correspondence, the collection includes material on Gaddum’s war-related research and provides an insight into the role of the medical scientist in World War II. Trained as a physiologist, Gaddum made his first major contributions to the field while working at the National Institute for Medical Research in the laboratory of Sir Henry Hallett Dale FRS (1875–1968). Together with Ulf von Euler ForMemRS, he discovered a previously unknown but vital vasodepressor substance in the brain and intestine (Substance P). Gaddum was able to establish the role of acetylcholine in sympathetic nerve transmission. His later research focused on the mode of action of drugs, and his studies on the effects of lysergic acid diethylamide (LSD) led to a better understanding of serotonin. 1 The laboratory notebooks in the collection chart the course of these discoveries: one day’s entry reads, ‘various unsuccessful attempts to show that sympathetic ganglia act by liberating acetylcholine’; 2 other notable entries include a 1953 self-experiment to record LSD’s physiological effects under the supervision of his research team. 3 Gaddum developed ever more sophisticated methods of testing the pharmacological activity of substances. Among his papers are many diagrams of apparatus and push–pull cannulae, devices for withdrawing fluids from tissue. Extensive correspondence on bioassay techniques shows how widely these methods were taken up by colleagues. Like many of his colleagues working in physiology during the 1940s, Gaddum was called upon to use his expertise in the field of gas warfare research. At Porton Down, he studied how the different dispersal properties of gases used in warfare determined their toxicity. In the company of Sir Joseph Barcroft FRS, Gordon Roy Cameron FRS and Sir Charles Arthur Lovatt Evans FRS, he advised the Porton group on both defensive and offensive war strategies.