Risk evaluation of complex environmental mixtures of polychlorinated dibenzo-p-dioxins and related halogenated aromatic hydrocarbons (polychlorinated dibenzofurans, azo- and azoxybenzenes, naphthalenes and some of the biphenyls) is currently carried out by measuring the concentration of each congener in the mixture and then multiplying every figure by its specific constant, toxic equivalency factor (TEF). All congeners are thought to produce highly similar effects albeit at different doses, and the TEFs are believed to represent the potencies of the congeners relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), considered the most toxic derivative of this class of environmental contaminants. Here we compared the acute toxicities of TCDD, 1,2,3,7,8-penta-, 1,2,3,4,7,8-hexa- and 1,2,3,4,6,7,8- heptachloro-dibenzo-p-dioxin in the most TCDD-susceptible (Long-Evans Turku AB; L-E) and the most TCDD-resistent (Han/Wistar Kuopio; H/W) rat strain. While L-E rats exhibited the expected rank order of sensitivities to the four dioxins, the higher chlorinated dioxins were more toxic than TCDD (in terms of acute lethality) to H/W rats, with the hexachlorodioxin showing the greatest potency. Even if the doses were adjusted according to the LD50 values, both biochemical and morphological effects elicited by the dioxins turned out to depend, often critically, on strain, congener or the interaction of these two determinants. These findings demonstrate that the dioxins have distinct profiles of acute toxicities and underscore the importance of response and test organism in defining the TEFs.