AbstractExperimental animals are often used as surrogate models in estimating human cancer risk from environmental agents when adequate epidemiological data are not available. Studies in experimental animals have usually evaluated the effects of exposure to single substances; however, humans receive combinations of exposures, to both initiators and promoters of carcinogenesis. Exposure to several agents may modify the carcinogenic process of one of them. For many agents, the relationship between dose and carcinogenic response depends on both dose‐rate and cumulative dose. For a given total dose, dose‐rate may affect carcinogenic potency both qualitatively (target organ) and quantitatively. The effects of dose‐rate are a function of total dose, species, and, most importantly, the mechanism by which the agent exerts its carcinogenic effect. Prediction of the effects of different dose‐rates of potentially carcinogenic agents can be based on knowledge of its mechanism of