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On the various days of the 5-day oestrous cycle of the rat, ovarian antral follicles were dissected out and grouped in five size classes. Four follicles of the same size class were homogenized jointly in medium, after which inhibin-like bioactivity, inhibin immunoreactivity and oestradiol-17β content were measured. In general, there was a significant correlation between immunologically and biologically active inhibin levels in the different size classes; overall correlation was 0·85 (n=87, P<0·00001). In the smallest antral follicles (classes 1 and 2) inhibin bioactivity was detected only during the first three days of the cycle. With increasing follicle size, inhibin bioactivity and immunoreactivity increased, with maximal activity present in the largest, i.e. preovulatory, follicles (class 5) during the last three days of the cycle (the day of oestrus denotes day 1 of the cycle). These results indicate that only follicles which reach the antral stage at oestrus, and are known to be recruited by the periovulatory FSH peak, acquire the potency to produce biologically active inhibin. This is the cohort of follicles from which selection of ovulatory follicles will normally take place.

In contrast to inhibin, follicular oestradiol-17β concentrations were negligible until the last days of the cycle when oestradiol-17β was present in follicles larger than class 2; levels increased with increasing follicle size and a maximal level was found in preovulatory follicles at pro-oestrus.

It is concluded that (1) there is a good correlation between follicular content of inhibin-like bioactivity and inhibin immunoreactivity and (2) there are differences between patterns of follicular levels of inhibin immunoreactivity and oestradiol-17β during the ovarian cycle: follicles destined to ovulate start to produce inhibin in volume classes 1 and 2 and only thereafter also contain oestradiol-17β. Follicles entering classes 1 and 2 at dioestrus-2 and -3 or at pro-oestrus do not produce inhibin and become atretic.

Journal of Endocrinology (1995) 146, 323–330

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