Mandibulofacial dysostosis is an autosomal dominant malformation incorporating a number of facial and hearing defects. It has been regarded as a symmetrical problem with a central causative mechanism of malformation. Patients with this syndrome occasionally show a tendency toward asymmetry from birth to adolescence. Rat models showing similar facial defects have already been described. These phenocopies developed when the pregnant dam was given a dose of 100,000 units of vitamin A on day 8.5 day of gestation. This report describes the effects of a dose of 50,000 units given to the pregnant dam on day 8.5. An asymmetric deformity was produced which was similar to that occasionally seen in man. Macroscopic and microscopic observations distinguish the departures from normal development of the affected rats. Possible causal mechanisms are discussed.