SummaryThe literature on the use of radioactive materials in various kinds of ecological work on small mammals is reviewed under three main headings–inert implants used as markers for tracing movements, external radioactive tags used for similar purposes, and metabolizable radionuclides introduced into the animals' bodies not only for direct tracing of movements of the marked individuals, but for indirect tracing by detecting radioactive excreta, and by automatic marking of offspring and ectoparasites, which can in turn be identified and traced. A comprehensive table summarizes the literature reviewed, and summarizes also the characteristics of the various radioactive isotopes used or suggested for use.Tests were carried out on various possible methods of using iodine‐131 as a marker for small mammals. Intraperitoneal injection of 131‐iodide as silver iodide or bound to an ion‐exchange resin gave an unsatisfactorily short biological half‐life for direct tracing but might be useful for detecting excreta. Silver 131‐iodide incorporated in epoxy resin and encapsulated in metal or plastic could prove to be a useful implant.Silver 131 ‐iodide paint was quickly removed from the animals' fur by grooming, but might be useful for insects.Silver‐131‐iodide–Araldite mixture on a leg ring proved useful, despite the inherent disadva