Hamster ovaries explanted at 21:00\p=n-\24:00 h on the day of pro-oestrus were incubated with microbial proteinase inhibitors until 10:30 h on the next morning and the ovulatory blocking effect of these inhibitors was examined. Amongst 11 proteinase inhibitors examined, talopeptin, a specific inhibitor for metallo-proteinases, and \ g = a \ \ x = r e q -\ MAPI, a specific inhibitor for serine and thiol proteinases, were the strongest blockers. These 2 inhibitors exhibited a chronological

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... ancy in their blocking effect on ovulation. S-SI, plasminostreptin, elastatinal, antipain and chymostatin, which are inhibitors for serine proteinases, partly but significantly inhibited ovulation. The results suggest that, in addition to a metallo-proteinase reported previously, a proteinase which is sensitive to \g=a\-MAPIis essential for the ovulatory process, and that serine proteinase(s) also participate in ovulation of the hamster ovary.