Lecture given in the memory of Rudolph Schoenheimer, by Otto Loewi Describes the work of Schoenheimer on using isotopes to trace the course and rate of metabolism; also the work of Urey on heavy hydrogen, which made his work possible; introduces Georg de Hevesy, Nobel prize winner in chemistry for his work on isotopes, to give a speech on 'The application of radioactive indicators to the study of metabolic processes'. nd