Further report on the interceptive STS 557 in baboons.

: STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4,9(10)-diene-3-one) or levonorgestrel was given orally as a single dose of 0.4 mg per animal. Treatment was performed immediately after the 6 h mating period. In 30 control cycles (18 animals) 9 pregnancies (30%) and in 26 cycles (17 animals) with levonorgestrel treatment 3 pregnancies (11.5%) were observed. In 46 cycles (21 animals) with STS 557 treatment only one pregnancy (2.2%) occurred (chi 2-test: P less than 0.01 as compared to control cycles). Therefore, STS 557 showed high interceptive activities in this nonhuman primate model. In contrast with levonorgestrel, STS 557 did not decrease the postovulatory rise of plasma progesterone.