Tripterygium wilfordii Hook f. is a medicinal plant which possesses anti-fertility activity in males and has been used in China for the treatment of rheumatoid arthritis, chronic nephritis and other pulmonary diseases for years. The effect of Tripterygium wilfordii extract on in vitro development in day 9 and day 10 mouse embryos was examined. The embryotoxicity of T. wilfordii extract was obvious on day 9 and day 10 embryos at doses of 50 and 100 g/ml, respectively. The embryonic abnormalities produced included abnormal branchial apparatus, open cranial neural tube and absence of forelimb bud in day 9 embryos. In day 10 embryos, the abnormality appeared first in the branchial apparatus. At higher doses, additional abnormalities appeared: abnormal optic and otic vesicles and twisted body axis in day 9 embryos; microcephaly, hematoma in cranial region and absence of hindlimb buds in day 10 embryos. There were reductions in final somite number, yolk sac surface area and axial length. Dead cells were present all over the embryo. The blood circulation of the visceral yolk sac of the abnormal embryos became defective. Ultrastructural analysis of the visceral yolk sac revealed abnormalities including decrease in the amount of microvilli, reduction in the number of storage vesicles, appearance of large vacant vacuoles and decrease in the number of pinocytotic invaginations on the apical surface of the yolk sac epithelium. The embryotoxic effect of T. wilfordiiextract was, however, destroyed by heat treatment of the extract at 80oC for 6 hours.