JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The National Institute of Environmental Health Sciences (NIEHS) and Brogan & Partners are collaborating with JSTOR to digitize, preserve and extend access to Environmental Health Perspectives.We review the experimental evidence for various shapes of dose-response relationships for carcinogens and summarize those experiments that give the most information on relatively low doses. A brief review of some models is given to illustrate the shapes of dose-response curve expected from them. Our major interest is in the use of dose-response relationships to estimate risks to humans at low doses, and so we pay special attention to experimentally observed and theoretically expected nonlinearities. There are few experimental examples of nonlinear dose-response relations in humans, but this may simply be due to the limitations in the data. The several examples in rodents, even though for high dose data, suggest that nonlinearity is common. In some cases such nonlinearities may be rationalized on the basis of the pharmacokinetics of the test compound or its metabolites. mulae"). Of special interest are the observed nonlinearities in dose-response relationships, and the possible explanations for them.Most models of the cancer process are constructed by describing (mathematically) a set of elementary biological processes which are supposed to be fundamental. The effect of doses of carcinogens on these elementary processes is generally assumed to be the simplest possible (e.g., described by a chemical reaction rate), but the dose-response relationship for the whole model will usually be as arbitrary as the assumptions made for these elementary processes. Many of the mathematical models suggested are expressions of the biological idea of a multistage process. We demonstrate how many different dose-response formulae can be obtained within this one framework, although such formulae might also arise from alternative theoretical frameworks. Consequently, to avoid confusion, it seems preferable to refer to currently used dose-response relationships by some descriptor of the mathematical formulae, rather than by a descriptor of the particular theory or model used to justify them. "Human Studies" and "Animal Studies" summarize those experimental observations on cancer induction which show most clearly the shapes of dose-response curves. Epidemiological studies and bioassays can use too few subjects to give direct information about doses at which excess tumor rates are below about 1%. Where large numbers of people or animals have been exposed to various levels of a carcinogen we obtain the closest approach to low doses. In addition, studying the outcomes of a large number of small experiments can give P...