Evidence obtained in the rabbit, an induced ovulator, has created a general view that the sperm capacitation ability of the oviduct cannot be modified significantly by ovarian steroids. The possibility that this may not be so in the reproductive tract of cyclic species has been investigated by timed transfer of eggs and spermatozoa into female hamsters maintained in different endocrine states. In estrous females, few native eggs were penetrated by 5 hours, but a majority were penetrated 6--6.5 hours after insemination of fresh epididymal spermatozoa directly into the ovarian bursa. This indicates that a time of approximately 5--6 hours is required for capacitation in the hamster oviduct alone. Up to 2 hours were needed for most eggs to be penetrated after their transfer to oviducts containing fully capacitated spermatozoa. Subsequent experiments using these criteria showed that the ability of the oviduct to support fertilization was affected significantly by the endocrine state of the female. It was not influenced noticeably by ovariectomy. However, whereas maximal levels were seen on days 1, 3, and 4, fertilization was reduced significantly (P less than 0.001) on day 2 of the estrous cycle when progesterone levels had been elevated for approximately 48 hours. Fertilization of eggs transferred to inseminated oviducts was also reduced comparable in ovariectomized hamsters injected with 3 mg of progesterone twice per day for 9--12 days, and in those that were 8--10 days pregnant. The prospects for complete suppression of capacitation in species with more prolonged cycles and the relevance of this for contraception and analysis of the factors responsible for capacitation are discussed.
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