The purpose of this study was attempted to investigate the number changes of the growing and mature follicles in ovary following repeats of pregnant mare serum gonadotropin(PMSG) treatments for superovulation in nulliparous rats. Thirty two rats(Sprague-Dawely, about 200~250 gm) were randomized into 4 groups. Control group rats were sacrified at estrus phase confirmed by vaginal smear. PMSG-treated group 1 rats, PMSG-treated group 2 rats and PMSG-treated group 3 rats were sacrified at 48 hrs after injection once with PMSG 25 IU, after 2 repeated injection by a week interval, and 3 repeated injection, respectively. The uteri and ovaries of rats were removed and weighed and then were observed grossly and serial sections of all ovaries and some sections of uteri by paraffin embedding were stained with H-E. Number of ovarian follicles about 3 grades of small, middle and large follicles from seondary and follicles were investigated by LM photographies of ovary preparations. The criteria of the small, middle, and large follicles were based as small follicle with preantral follicles with 2~4 layers of granulosa cells surrounding the oocyte, as secondary follicles with more than 5 layers of granulosa cells and early signs of antral cavity or with small clefts on either side of the oocytes, and as tirtiary follicles with a single medium sized antral cavity or large well-formed antral cavity, respectively. In gross findings, the wall of the uteri in control group were thin, and those in 3 PMS-treated group were markedly thickened and some uterine lumen of those filled with fluid. In histological findings, the walls of the uteri from 3 PMSG-treated groups were hypertrophied and their blood and lymph vessels were dilated than those of control group. The ovaries fo 3 PMSG-treated groups were more increased in size and the cortexes were more developed and increased in width but there are no difference of development and changes in 3 PMSG-treated groups. The weight of the uteri and ovaries per rat in PMSG -treated group 1, 2 and 3 were a, pp.ared to be significantly increased 171.4$pm$47.6%, 162.3$pm$43.9%, 206.9$pm$30.4%, respectively than those of control groups. The mean number of follicle per ovary in control group were a, pp.ared to be 17.1$pm$3.5, 46.2$pm$14.5, and 74.3$pm$22.7 at large, middle and small follicles, respectively and total number of these 3 grade follicles per ovary were a, pp.ared to be 137.7$pm$31.7. The mean number of follicle per ovary in PMSG-treated group 1 were a, pp.ared to be 25.6$pm$7.3, 78.1$pm$29.9, and 83.2$pm$34.0, at large, middle and small follicles, respectively and total number of these 3 grade follicles were a, pp.ared to be 187.5$pm$58.8. The mean number of follicle per ovary in PMS-treated group 2 were a, pp.ared to be 21.9$pm$5.2, 67.8$pm$16.8, and 68.0$pm$14.9 at large, middle and small follicles, respectively and total number of these 3 grade follicles were a, pp.ared to be 157.7$pm$26.2. The mean number of follicle per ovary in PMS-treated group 3 were a, pp.ared to be 21.7$pm$4.8, 61.5$pm$17.0, and 59.7$pm$16.2 at large, middle and small follicles, respectively and total number of these 3 grade follicles were a, pp.ared to be 143.5$pm$29.6. The number of follicles in PMSG-treated group 1 a, pp.ared to be more number than other 2 PMSG-treated gruops and tended to be decreased by frequency of PMSG-treatment.