Studies on the mechanisms of chemical toxicity carried out using knockout mice lacking genes of enzymes for drug metabolism or nuclear receptor proteins were reviewed, and the studies were compared with the respective conventional mechanistic studies. While the toxicity of many hazardous chemicals was observed only in wild-type or knockout mice, which clearly showed that their toxicity was involved in the enzyme or receptor, some chemicals exhibited the same degree of toxicity in two genotypes, i.e., in both the wild strain and knockout mice, demonstrating that the enzymes or receptors are not involved in their toxicity. The use of genetically-modified animals presents not only the advantage of simultaneous evaluation of toxicity endpoints and mechanisms, but also suggests significant benefits over conventional methods using several chemicals to elucidate toxicity mechanisms. Elucidation of the mechanism of toxicity will provide useful information for risk assessment, and the use of genetically-modified animals for this purpose will lead to the advancement of this assessment.