Effect of STS 557 on semen composition, fertility and sexual behaviour of male rabbits.

Fertility control of male rabbits was achieved by daily oral administration of STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4.9(10)-diene-3-one) over a period of 8 weeks. In doses of 10 and 20 mg STS 557 per animal per day, fertility inhibition was accompanied by a decrease of mean number of mounts and ejaculations. Furthermore, semen volume, number of sperm per ejaculate, and sperm motility were reduced. At a dose of 5 mg STS 557 per day, male sterility was associated with reduction of sperm motility and semen fructose content. Libido, semen volume, sperm number, and sialic acid content in semen remained unaffected. Development of male contraceptives on the basis of chronic progestin treatment without concomitant androgen supplement may thus be possible.