Whole embryo rotated culture technique was used to investigate the toxicity of combination of selenium, fluoride and arsenic on rat embryos at day 9.5 of gestation. The result of factorial analysis (3 x 3 x 3) showed that the main effect of combination of selenium, fluoride and arsenic on the developmental toxicity was synergistic. The mixtures with different level of these three chemicals in combination could result in different developmental toxicity. The low level combinations mainly caused teratogenic effect, and the high level combinations(selenium 2.0 micrograms + fluoride 10 micrograms + arsenic 1.0 microgram/ml culture media) caused lethal effect. The results suggested that the disorders of yolk-sac placenta in structure and function were one of teratogenic mechanisms for the combination of selenium, fluoride and arsenic.