Changes in the pattern of follicular growth and development, and the associated endocrine changes, were examined in prepubertal heifers approaching their first ovulation. Ten, age-matched (+/- 3 days), Spring-born Hereford heifers were examined daily by transrectal ultrasonography for 17 days beginning 12 weeks before the first ovulation, and daily from just before the first ovulation until the completion of one normal duration ovulatory cycle. On each day of ultrasound examination, the position and diameter of corpora lutea and follicles > or = 3 mm in diameter were recorded, and one blood sample was collected. Blood samples were also collected every 15 min, for 12 h, at 20, 12 and 4 weeks before the first ovulation, to assess the pulsatile nature of LH and FSH secretion. The first ovulation occurred at 56.0 +/- 1.2 weeks of age, at a body weight of 391.9 +/- 12.0 kg. kg. Waves of follicular development, similar to those of adult cows, were seen at all ages, and in all heifers, the first ovulation was followed by an ovulatory cycle of short duration (7.7 +/- 0.2 days) and then by a normal duration ovulatory cycle (20.3 +/- 0.5 days). The maximum diameter of the dominant, or largest subordinate, follicles did not increase as the first ovulation approached, or during the subsequent ovulatory cycles. Similarly, there were no differences between follicle growth rates (1.4 +/- 0.1 mm day-1) or regression rates (1.2 +/- 0.1 mm day-1) as the animals matured, and the interwave interval increased up to the first ovulation.(ABSTRACT TRUNCATED AT 250 WORDS)