The effects of maternal dietary biotin deficiency on hamster embryos were examined by adding different amounts of avidin (0, 10, 50, 100 or 1000 mg/kg diet) to a semipurified commercial diet during the entire period of gestation. On d 10 of gestation, reduced dietary biotin resulted in a high incidence of resorbed and dead embryos. In addition, both the crown-rump length and head length of dietary biotin-deficient embryos were lower, and their digit development was retarded. These embryos were characterized by pericardial cavity enlargement (40%) and zig-zag closure line of the neural tube (44%). Some embryos exhibited abnormalities of the craniofacial region and tail. On d 14 of gestation, embryonic growth retardation, morphological abnormalities and skeletal defects were seen in the dietary biotin-deficient group (fed 100 mg avidin/kg diet). The striking abnormalities were cleft palate, micromelia, micrognathia and rib deformities in approximately 10% of the fetuses. Histological examination of the placentae revealed some differences in the spongiotrophoblast and labyrinth layers between the control and dietary biotin-deficient groups. The teratogenic effect of dietary biotin deficiency previously observed in mice was confirmed in hamsters.