In this study, the efficacy of human chorionic gonadotropin (hCG) and gonadotropin releasing hormone (GnRH) administered to advance the ovulation in oestrous in mares was examined. Thoroughbred mares (36) were divided into 3 groups. After the detection of (35 mm follicle in diameter, with the use of ultrasound, 3000 IU hG, 40mg busereline, GnRH and 2 mi placebo were injected to the mares in groups I, 2 and 3 respectively. Follicles were monitored every 12 hr with ultrasonography until the occurrence of ovulation. The mares were mated naturally every other day by the end of oestrus. Pregnant mares were determined using B-mode real time ultrasound on day 20 after last mating. There was a statistical significance of(P<0.0 If between groups 1 and 3, and also 2 and 3 on the average ovulation time and numbers of mating required per conception. On the other hand, the differences of the pregnancy rates among the groups were not significant. In conclusion, hCG and GnRH could be used for inducing ovulation in cyclic mares.