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Preovulatory bovine follicles (n = 58) were collected at different times after the onset of standing oestrus when cows would allow mounting until shortly before ovulation, which occurred 24 ± 1·4 h after the peak level of LH in the peripheral blood. Non-atretic antral follicles (n = 71) of 3–20 mm were also collected from cows during the luteal phase of the oestrous cycle. The follicular fluid was aspirated for the radioimmunoassay of oestradiol-17β, progesterone and testosterone. The follicular wall was examined micromorphologically. Follicular fluid steroid levels were compared in 2-h periods relative to the LH peak. The development of the preovulatory follicles from onset of oestrus to ovulation can be divided into four phases. Phase 0 (after onset of oestrus but before LH surge) was characterized by a high level of oestradiol (6·05 µmol/l); the levels of progesterone and testosterone were lower (0·387 and 0·165 µmol/l respectively) but higher than in non-atretic luteal follicles of similar size. The theca interna (TI) was wider and the membrana granulosa (MG) cells were larger than those of non-atretic antral follicles. During phase 1 (0–6 h after the LH peak) oestradiol remained constant but at a lower level, progesterone increased (0·727 µmol/l) and testosterone was higher from 0 to 2 h after the LH peak (0·241 µmol/l). The TI was 40% wider, whereas the size of the MG cells remained the same. In phase 2 (6–20 h after the LH peak) the level of oestradiol dropped rapidly during the period from 6 to 10 h, that of progesterone decreased to the same level as in phase 0 and that of testosterone was low (0·031 µmol/l). The width of the TI decreased to that of preovulatory follicles in phase 0 and the MG cells were slightly larger. In phase 3 (20 h after the LH peak until ovulation) the level of oestradiol decreased further (0·461 µmol/l) and that of testosterone remained low. Progesterone increased to the highest levels observed (1·51 µmol/l), however, and this coincided with a 39% increase in the size of the MG cells, whereas the width of the TI remained the same as in the preceding phases 0 and 2. In phase 3 the basement membrane began to disintegrate and phagocytic cells could be observed. This points to a simultaneous functional and morphological luteinization.

It is suggested that these changes in follicular steroid levels and micromorphology are regulated by the preovulatory LH peak.

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