Oral administration of 400 mg/kg of 13-cis retinoic acid to 9 day pregnant mice gives rise to important maxillofacial malformations. The first manifestation of teratogenic effect is an increase of density of cell death arising in the dorsal part of the first two branchial arches at day 9.5. These two arches become hypoplastic at days 10 and 11, and the preskeletal anlagen appear too late in comparison to control embryos. Meckel's cartilage is too curvilinear and medially situated. Pre-ossicular and pre-mandibular blastemata develop with spatial distortions which are well analyzable at days 16 and 17