The present study was designed to study the follicular population dynamics followed or not by treatment with different doses of the GnRH antagonist in sheep. A total of 18 ewes were submitted to short-term oestrus synchronization protocol (Oliveira et al. 2009 Proc. Braz. Congr. Anim. Reprod.). The animals were 2 or 3 years old, multiparous, and had a body score of 3 to 3.5. On Day 7 after ovulation of synchronized oestrus, females were randomly divided into groups (n=6/group) according to the dose of the gonadotrophin-releasing hormone (GnRH) antagonist (Firmagon®, Ferring Pharmaceuticals) subcutaneously administered: G-control: placebo treatment (administration of saline solution); G-lower dose: 215 µg/kg; and G-higher dose: 235 µg/kg of bodyweight. B-mode ultrasound exams of the ovaries were conducted daily from 1 day before treatment with GnRH antagonist until the females showed oestrous behaviour. Ultrasound equipment (MyLab Vet®, Esaote) was used coupled to a transrectal linear transducer with a frequency of 6 and 8MHz to assess the ovarian population. Data were compared between groups, evaluation days, and their interaction by ANOVA with post hoc using Tukey’s test (P<0.05). There was no interaction (P>0.05) between the studied effects (treatments and evaluation days). The number of small follicles (2–3.49mm) was higher (P=0.0002) in the G-lower dose (5.4±0.4) compared with the G-control (4.1±0.3) and G-higher dose (3.5±0.2). The number of large follicles (≥4.5mm) was lower (P=0.01) in the G-higher dose (0.2±0.0) compared with the G-control (0.5±0.1) and G-lower dose (0.4±0.1). The number of medium follicles (3.5– 4.49mm) and the average diameter of the follicles in the 3 categories of diameter did not differ (P>0.05) between groups. The number of medium follicles differed (P=0.0131) between Days 8 and 15 after synchronized oestrus ovulation. The number of large follicles on Day 6 differed (P=0.0002) of Days 8, 9, 10, 11, 12, 16, and 17. The average diameter of medium follicles differed (P=0.0095) between Days 8 and 10. The number of small follicles and the average diameter of small and large follicles did not differ (P>0.05) between days. In conclusion, the administration of the GnRH antagonist at a higher dose in sheep suppressed the development of large tertiary or antral follicles, whereas at a lower dose, it led to an increase in the population of small follicles.