The incidence of malformation is increased in infants of hyperthyroid or hypothyroid woman. Although many papers reported that the fetus is insulted from maternal thyroid hormone, the placenta (maternalfetal barrier) is not yet fully developed before 11.5 days of gestation in rat embryos, suggesting the effect of thyroid hormone on early rat embryogenesis. This study was, therefore, undertaken to investigate whether excess or lack of thyroid hormones would affect early embryogenesis in rat embryo culture. Malformations including open neuropore and microencephaly were observed in 10 of 30 embryos incubated in hyperthyroid serum, and in 12 of 42 cultured in T₃-enriched normal serum. Similar malformations were observed in 14 of 42 embryos cultured in hypothyroid serum and in 10 of 30 cultured in hypothyroid serum supplemented with T₃. The frequencies of these malformations were significantly higher than in the control embryos (0 in 72 embryos) cultured with normal rat serum. These results suggest that the maternal thyroid status might play an important role for the complication of fetal malformations during early gestational period.