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We reported previously that active immunization of heifers using a synthetic peptide-based inhibin vaccine (bI
(1–29)Tyr30) can enhance ovarian follicular development and ovulation rate during spontaneous oestrous cycles. To extend this study, we investigated the effect of inhibin immunization more closely by monitoring plasma hormone profiles and ovarian activity in bI(1–29)Tyr30-immunized and control (ovalbumin-immunized) heifers (n = 6 per group) over three consecutive oestrous cycles, which were synchronized and shortened by administering a PGF2 analogue at intervals of 14 days. Blood samples were collected at 2–8 h intervals for 40 days and the ovaries were examined daily using ultrasonography. Repeated-measures anova showed that inhibin immunization significantly increased plasma FSH concentration (by 52% overall; P < 0.01) and ovulation rate (by 58%; P < 0.01). Both immunized and control heifers showed the same cyclic pattern of plasma FSH (treatment x time interaction; not significant), indicating that the increase in plasma FSH was sustained throughout the cycle. Immunization did not affect the concentration or pattern of secretion of LH, oestradiol or progesterone and had no influence on the timing of the LH surge or ovulation after PG injection. While inhibin immunization increased the number of 'large' (i.e. growing to 
10 mm diameter) follicles that developed during both the preovulatory (by 90%, P< 0.02) and postovulatory (by 190%, P< 0.01) period, there was no difference between groups in the temporal pattern of growth or regression of large follicles or of corpora lutea. These observations confirm a physiological role for ovarian inhibin as a component of the ovarian feedback mechanism controlling FSH secretion in heifers, and support the hypothesis that active immunization of heifers against inhibin enhances ovarian follicular development and ovulation rate by promoting a sustained increase in pituitary FSH secretion.
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