In sheep, the injection of a single dose of 1.5 mg of the GnRH antagonist Teverelix (Antarelix^™, Zentaris, Frankfort, Germany) eliminates large dominant follicles and increases the number of smaller follicles (2–3 mm in size) in a short period of time (Lopez-Alonso et al., 2003. Reprod. Abstr. Ser., 30:71). This treatment would be beneficial for increasing the efficiency of ovarian stimulatory protocols, since embryo output is enhanced in the presence of a high number of small follicles, coincidentally with the absence of large follicles, at starting the FSH treatment. However, possible effects of this single high dose of GnRH antagonist on the capacity of follicles to develop in response to FSH treatments has not been determined. In this study, we have characterized patterns of follicular development during a superovulatory treatment with purified ovine FSH (Ovagen^™, ICPbio, Auckland, NZ) in sheep treated i.m. with 1.5 mg of Antarelix (n = 6) or saline (n = 4) on Day 9 after the insertion of a progestagen sponge (Chronogest®, Intervet Int, Boxmeer, The Netherlands). Ewes were superovulated with eight decreasing doses (1.5 × 3, 1.25 × 2 and 1 × 3 mL) of Ovagen^™ injected twice daily from Day 12 of sponge insertion. Number and size of all >2-mm follicles was determined by transrectal ultrasonography just prior to every FSH injection from the first dose to the withdrawal of progestagen sponges. At the start of the gonadotrophin treatment on Day 12, ewes treated with GnRH antagonist showed a higher number of 2–3 mm follicles than control ewes (16.2 ± 3.8 v. 5.3 ± 0.3, P < 0.05), and a lower number of >4 mm follicles (2.2 ± 0.5 v. 5.0 ± 0.6, P < 0.01). Thereafter, administration of Ovagen^™ induced a significant rise in the number of >4 mm follicles at sponge removal in both groups (P < 0.0005 for treated ewes and P < 0.01 for controls). This number was higher in females treated with GnRH antagonist than in control sheep, although differences did not reach statistical significance (19.3 ± 3.8 v. 12.7 ± 5.4). Current data confirm that administration of a single dose of GnRH antagonist decreases presence of large follicles and increases the number of smaller follicles at the first day of FSH injection. This pre-treatment does not affect competence of follicles to grow in response to superovulatory FSH treatments. Moreover, the number of preovulatory follicles at sponge removal was higher than in untreated ewes. We conclude that the pre-treatment with a single dose of 1.5 mg of Antarelix^™ on Day 9 of sponge insertion, three days before starting the FSH treatment in a superovulatory protocol, might increase the ovarian response and, thereafter, the yields of follicles.