The U.S. Environmental Protection Agency has undertaken the task of preparing a scientific reassessment of the health risks of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin and chemically similar compounds collectively known as dioxin. Significant advances have occurred in the scientific understanding of mechanisms of dioxin toxicity, of the carcinogenic and other adverse health effects of dioxin in people, of the pathways to human exposure, and of the toxic effects of dioxin to the environment. The health assessment document consists of three volumes. Volume 1 describes the disposition and pharmacokinetics; mechanism(s) of action; acute, subchronic, and chronic toxicity; immunotoxicity; developmental and reproductive toxicity; and carcinogenicity of TCDD in animals.