Effects of levonorgestrel and STS 557 on testis in rodents.

: In immature male Wistar rats levonorgestrel and STS 557 (17 alpha-cyanomethyl-17 beta-hydroxyestra-4,9(10)-diene-3-one) reduced testicular growth, testicular DNA contents and plane of tubular cross-section in a dose dependent manner. In this respect, STS was less active than levonorgestrel. In immature castrated male rats the LH-suppressing effect of STS 557 was about 15 times lower than that of levonorgestrel. The results suggested that peripheral inhibitory effect of the two progestins was coupled with central LH suppression. On the other hand, in mating tests with male hybrid mice STS 557 possessed high fertility inhibiting activities in comparison to levonorgestrel and chlormadinone acetate. Limited dissociation has been shown between fertility inhibition and mating rate. All changes were reversible. Findings are discussed in view of development of male contraceptives.