Induction of secondary ovulations during the first two weeks post-ovulation could increase progesterone concentrations in pregnant mares predisposed to pregnancy loss due to low progesterone concentrations, such as those carrying mule pregnancies. The objective of this study was to evaluate the effectiveness of deslorelin to induce follicular growth, ovulation and secondary CL formation in mares. Ten adult, healthy mares were used on several occasions for a total of 30 cycles. In the experimental group (n=24 cycles) the mares were treated with 600 μg of deslorelin acetate every 12 h for up to five days, starting when a follicle ≥19 mm was first detected by ultrasonography during diestrus. When the follicle reached 35 mm a bolus injection of 1.8 mg deslorelin was administered to induce ovulation. In the control group (n=6 cycles) the mares were treated with injectable water every 12 h for 5 days starting when a follicle ≥ 19 mm was first detected during diestrus. Daily ultrasonographic observations were continued until day 25 post-ovulation to determine the occurrence of ovulation and secondary CL formation during diestrus. In 19/24 (79%) of cycles treated with deslorelin a follicle grew to ≥ 35 mm, while in the control group it only did so in 1/6 (17%) cycles (p˂0.05). In the treatment group, 4/19 mares thatdeveloped large follicles ovulated and formed a secondary CL. These represented 21% of the mares that developed a large follicle and 17% of the total number of treated mares. No secondary diestrus ovulation occurred in the control cycles. A sustained increase in progesterone concentrations occurred in the cycles with secondary ovulations after treatment with deslorelin, suggesting continued progesterone production by both the primary and the secondary CL. In some of the mares that did not ovulate a sharp but transient increase in progesterone concentrations occurred after the administration of 1.8 mg deslorelin, possibly reflecting stimulation of the primary CL. It is concluded that deslorelin is effective to induce follicular growth up to a preovulatory size in diestrous mares. However, a low proportion of them ovulated and formed a secondary CL after the final injection of deslorelin, suggesting that another alternative, such as hCG administration should be tested to induce ovulation of the follicles stimulated by deslorelin during the first five days post-ovulation in mares. Acknowledgements: Project financed by PAPIIT project IN225923, DGAPA, UNAM, México.