Recent advances have been made in the characterization of a number of transgenic animal models. These animal models have provided a powerful toxicological tool for studying in vivo chemical effects and have increased our understanding of the role of specific genetic alterations as predisposing factors for chemical carcinogenesis. The goal of this symposium was to introduce the development of transgenic animals and the utilization of trans genies in toxicology research focusing on understanding tissue- specific mutation, chemical effects, and cancer. The production of transgenic animals, including gene insertions and gene knockouts, and the utilization of transgenic technology for studying multistage carcinogenesis and tumor suppressor genes are described. Data on the application and implications of transgenics as a genetic endpoint are also discussed. The use of transgenic animals in toxicology should improve our understanding of the role of specific genetic alterations in the carcinogenic process and lead to improved estimations of human health risks.