The aim of this study was to determine the effect on ovarian follicular growth and atresia, of acute treatment with either 100 mg of progesterone (n = 10), 200 mg of progesterone (n = 10), 10 mg of oestradiol + 100 mg of progesterone (n = 10), 10 mg of oestradiol (n = 10) or no treatment (n = 10), given on Day 10 of a 17-day treatment with a norgestomet implant in randomly cycling Bos indicus heifers. The fate of the dominant follicle on Day 10, emergence of the new cohort of follicles and the intervals from implant removal to ovulation were recorded by ultrasonography. Plasma concentrations of Luteinizing hormone (LH), progesterone and oestradiol were determined during the time when the norgestomet implant was in place. All treatments resulted in the emergence of a new cohort of follicles within 5 days of administration. The day of emergence of the ovulatory follicle tended to be delayed after treatment with 100 mg of progesterone (2.7 +/- 0.3 days after treatment), 200 mg of progesterone (3.7 +/- 0.5 days after treatment), 10 mg of oestradiol + 100 mg of progesterone (4.4 +/- 0.2 days after treatment) and 10 mg of oestradiol (4.6 +/- 0.4 days after treatment) compared to control heifers (1.4 +/- 1.4 days after time of treatment). The mean interval from implant removal to onset of oestrus was significantly shorter after treatment with 100 mg of progesterone (38.4 +/- 2.6 h) than after treatment with 200 mg of progesterone (61.5 +/- 3.9 h) but otherwise, the mean interval from implant removal to onset of oestrus did not differ. Oestrus synchrony, measured by the sample standard deviation of oestrus onset, was tighter in all treatment groups compared to untreated control heifers. The mean interval from implant removal to ovulation did not differ significantly between groups. The synchrony of ovulation, measured by the sample standard deviation of the interval from implant removal to ovulation, was significantly tighter after treatment with 100 mg of progesterone, 200 mg of progesterone and 10 mg of oestradiol compared to control heifers. Treatment with 10 mg of oestradiol resulted in ovulation in seven of 10 heifers before implant removal, three of which failed to ovulate after implant removal. Progesterone administered on Day 10 lowered plasma LH concentrations (P < 0.05), whereas treatment with oestradiol caused a surge of LH and ovulation. Progesterone administered with oestradiol prevented the LH surge. A combination treatment of oestradiol and progesterone given on Day 10 of a 17-day norgestomet treatment in a range of follicular states resulted in the consistent emergence of a new cohort of follicles which included the eventual ovulatory follicle.