Abstract: The present study was aimed at oestrus synchronization by applying progestagen-impregnated intravaginal sponges for either 7 or 12 days to lactating goats during the breeding season and at the determination of the effects of post-mating GnRH injection on fertility. The study was conducted in 80 hair goats aged between 2-5 years. The goats were allocated to two equal groups, one of which was applied short-term intravaginal sponge treatment and the other long-term treatment. The duration of intravaginal sponge treatment was 7 days in the short-term group (ST, n = 40) and 12 days in the long-term group (LT, n = 40). Furthermore, on the day of intravaginal sponge removal, the goats received intramuscular injections of 400 IU of PMSG and 0.075 mg of cloprostenol. In both groups, 12 h after the removal of the vaginal sponges, 10 fertile bucks were introduced into the flock for oestrus detection twice a day and goats which were determined to be in oestrus were hand-mated to the assigned bucks. Immediately after mating, the goats included in each of the ST and LT groups were randomly assigned to two subgroups, referred to as ST1 (n = 18), ST2 (n = 18), LT1 (n = 18) and LT2 (n = 18). The subgroups ST1 and LT1 were maintained as controls whilst the subgroups ST2 and LT2 were administered with 5 mcg of bucerelin acetate immediately after mating. In the present study, the times of oestrus onset and oestrus rates in the ST and LT groups were determined as 33.3±1.4 and 35.0±1.4 h and 94.7 and 97.2%, respectively. The differences observed between the two groups for the time of oestrus onset and oestrus rate were statistically insignificant (p>0.05). In the subgroups ST1, ST2, LT1 and LT2, pregnancy rates were determined as 55.5, 50, 50 and 55.5%, respectively whilst the parturition rate of all four subgroups was 100%. Furthermore, litter sizes were detected as 150, 200, 188 and 170% in the subgroups ST1, ST2, LT1 and LT2, respectively. The differences observed between the subgroups for pregnancy rate, parturition rate and litter size were statistically insignificant (p>0.05). In conclusion, it was demonstrated in the present study that GnRH administrations immediately after mating to lactating goats in which oestrus synchronization was performed by means of short and long-term intravaginal sponge treatment during the breeding season did not improve pregnancy rate, parturition rate and litter size.