Craniofacial malformations were induced in 256 embryos from 32 pregnant rats by a single intraperitoneal injection of 10 mg/kg etretinate at 8.5 days gestation. The litters developed several malformations including microtia, low set and dorsally placed outer ears, defective middle ear ossicles, short cochleas, defectively differentiated Meckel's cartilages, micrognathia, rudimentary malar bones, lateral facial clefts, fistulas, and skin tags, all of which tissues are derivatives of the first and second branchial arches. The teratogenically induced syndrome shows similarities to the mandibulofacial dysostosis syndrome in man. The defects were accompanied by a change in the histochemical differentiation and location of various enzymes in the craniofacial tissues.