The synthetic steroid STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4, 9-dien-3-one) was tested in gravid rabbits from days 5 to 25 p.c. It was the aim of the experiments to investigate any embryotoxic and/or teratogenic effect of STS 557. The compound did not show any effects on the mothers nor on the fetuses. Levonorgestrel was tested as standard.