Current study determined the effect of two different single subcutaneous doses (1.5 and 3 mg) of GnRH antagonist (GnRHa) on pituitary and follicular function in non-lactating cyclic ewes. Both doses abolished the pulsatile secretion of luteinizing hormone (LH) for at least 3 days and decreased mean LH concentration during 6 days (0.64 +/- 0.09 for control and 0.54 +/- 0.05, P < 0.005, and 0.46 +/- 0.02, P < 0.00001, for 1.5 and 3 mg, respectively). Supply of GnRHa decreased the number of large dominant follicles, so the total number of smaller follicles, 2-3 mm in size, increased in both treated groups from day 0, reaching its maximum at day 2 in ewes treated with 1.5 mg (19.83+/-1.05 versus 5.83 +/- 0.50 in the control, P < 0.005) and at day 4 in sheep treated with 3mg (18.67 +/- 0.65 versus 5.50 +/- 0.65 in the control, P < 0.0001). However, the analysis of follicular function in terms of inhibin A indicated a possible effect of the higher dose of GnRHa on follicular function. The pattern of inhibin secretion in the group treated with 3mg of GnRHa decreased after the first 48 h, reaching its lowest value on day 4.5 (182.59 +/- 3.75 to 140.28 +/- 9.91 pg/ml, P < 0.05) concentration significant lower than control sheep (171.93 +/- 6.21 pg/ml, P +/- 0.01) or treated with 1.5 mg (168.04 +/- 7.16 pg/ml, P+ /- 0.05). Hence, the use of 1.5 mg would be more suitable to induce the presence of a high number of follicles able to grow to preovulatory sizes.