Severe congenital malformations have been associated with the inadvertant use in early pregnancy of a new dermatological drug, isotretinoin. We present proposals for the pathogenesis of this embryopathy based on the study of animal models. The characteristic malformations of the face, thymus, and great vessels were induced in mice by prenatal exposure to the drug during the early somite stages of development. From histological examination of mouse embryos it was shown that the drug directly interferes with the development of cranial neural crest cells. Subsequent deficiency of crest cell-derived mesenchyme adequately explains most of the observed malformations. Rat embryo culture studies showed that, when used at concentrations of 500 ng/ml, both isotretinoin and its main metabolite in the human, 4-oxo-isotretinoin, induce malformations similar to those seen in vivo. Since during normal repetitive dosing in the human the mean trough blood concentration of isotretinoin ranges from 132 to 196 ng/ml, while 4-oxo-isotretinoin ranges from 610 to 791 ng/ml, it is likely that the metabolite plays a major role in the induction of the isotretinoin embryopathy.