Holstein heifers were given 5 injections (twice/day) of 10 ml charcoal-extracted bovine follicular fluid (bFF; N = 6) or 10 ml saline (N = 5) beginning 12 h after the onset of oestrus. Blood samples were collected for determination of plasma concentrations of FSH, LH, progesterone and oestradiol-17 beta. Treatment with bFF suppressed the secondary FSH surge (P less than 0.01). Cessation of bFF injections was followed by a rebound period during which FSH was elevated compared with controls (P less than 0.01). Daily ultrasonographic examinations revealed that follicular growth occurred in waves, with 4 of 5 control heifers exhibiting 3 waves and the other 2 waves. In contrast, 5 of 6 bFF-treated animals exhibited 2 waves and the other 3 waves. Appearance of follicles in the first wave was delayed in bFF-treated heifers (Day 3.3 +/- 0.3 compared with Day 1.4 +/- 0.2; P less than 0.0001) and appearance of the dominant follicle of the first wave was delayed (Day 4.5 +/- 0.3 compared with Day 1.8 +/- 0.2; P less than 0.0001). Follicles in the second wave appeared later in animals treated with bFF (Day 12.7 +/- 0.4 compared with Day 10.4 +/- 0.6; P less than 0.01), and the dominant follicle of this wave also appeared later (Day 13.0 +/- 0.5 compared with Day 10.6 +/- 0.5; P less than 0.01). Oestradiol-17 beta increased during the early luteal phase, but this increase occurred later in heifers treated with bFF (peak concentrations on Day 6.3 +/- 0.6 compared with Day 4.2 +/- 0.2; P less than 0.05). LH, progesterone and cycle length were not affected by bFF. Delayed follicular growth associated with suppression of FSH suggests that the secondary FSH surge is important in the initiation of follicular development early in the bovine oestrous cycle, and thus may play a role in the regulation of ovarian follicular dynamics.