The objective of this trial was to determine the efficacy of prostaglandin injections on different days of estrous cycle 10 induce ovarian activity of ewes during the breeding season. Vaginal sponges containing 30 mg fluorogestene acetate (FGA) were inserted into the vagina of the ewes tor 12 days, 0,294 mg Tnaprost tromethamine, an analogue of PGF_2α. was intramuscularly injected to all ewes one day betore the withdrawal of sponges in three groups. Estrous of each ewes were followed using with teaser rams with the 6 hour interval and mean ovulation time were determined based on the completing of estrous Animals were divided into three groups according the time of end of estrus. 0.294 mg Triaprost tromethamine were injected on day 3 (Group I), 6 (Group 11} and 9 (Group III) after ovulation. Afterwards, PMSG at the dosage of 400 IU were injected to all of the ewes at the moment of prostaglandin injection. PGF_2α-estrus, PGF_2α-ovulation interval were significantly (P<0.05) shorter in the group I than in the groups II and III. Plasma progesterone level at the moment of PGF,(I injections was statistically (p<0.05) lower in the group l than in the group II. On the other hands, estrus response and pregnancy rates were not statistically different (p>0.05) among the groups. As a result of in this study, injection of PGF_2α on day 3 after ovulation was found to be effective to induce ovarian activity in ewes.