The United States Environmental Protection Agency initiated a reassessment of the risk of exposure to dioxin because of the evolution of scientific thinking as well as the publication of several key experimental studies. This involves a literature reevaluation as well as laboratory studies in both health and ecological risks. One of the major goals is development of a biologically based dose response model. Directed health research involves comparison of sensitive responses in rats and mice, following both acute and chronic exposure, such as enzyme induction in multiple tissues, phosphorylaton responses, tissue dosimetry, and effects on the immune system. The relative potency of 16 dioxin-like chemicals is being compared following chronic exposure in mice for a number of responses in several tissues to better estimate TEFs following low-level, repeated exposures. In addition, the levels of mRNA for induced enzymes is being correlated with the total TEQ in human livers from the general population.