The issue of the developmental toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds has been the subject of two recent reviews (Morrissey and Schwetz, 1989; Couture et al., 1990a). There is little doubt that TCDD is one of the most potent developmental toxins known, yet its production of frank structural malformations in species other than in the mouse are poorly described. The objective of the review is to critically address the role which TCDD and its approximate isostereomers have in causing a wide array of developmental effects in various species, including some very recent results. The bias of the author is that the teratogenic response of the mouse is a reflection of extreme sensitivity of the species to the induction of frank teratogenic responses in two epithelial tissues. That is, that the mouse is an outlier in the field of developmental toxicity, possibly in parallel to the exquisite sensitivity of the guinea pig vs the resistance of the hamster to the lethal effects of TCDD, or in the resistance of haired rodents to the induction of chloracne. The crucial point is that most species respond similarly to TCDD; for any given endpoint outliers will exist. However, no species is an outlier for all responses. In terms of developmental toxicity, essentially all species critically examined to date demonstrate potent developmentally toxic effects following exposure to TCDD and related chemicals. Relatively low doses to the dam (varying within an order of magnitude) result in embryo/fetal toxicity. The actual induction of terata is an extremely rare response. (Copyright (c) 1991 Cold Spring Harbor Laboratory Press.)