A delicate balance of steroid and gonadotrophic hormones is essential for intrafollicular oocyte maturation and successful fertilization and embryonic development. Previous studies have demonstrated that a superovulatory dose of pregnant mare serum gonadotrophin (PMSG) has excessive gonadotrophic activity and alters intrafol1icular steroid hormone levels. In a series of four experiments, the morphology of oocytes and embryos retrieved from immature rats, treated with either a low or high dose of PMSG, and mature, cycling rats was compared to determine whether a superovulatory dose of PMSG has an adverse effect on oocyte maturation and subsequent fertilization and embryonic development in immature rats. Morphological criteria for the assessment of intraoviductal oocyte aging were established in the first experiment. During intraoviductal aging, progressive morphological changes directed by the intrinsic developmental program of the oocyte were observed. Further alterations in morphology were attributed to abnormalities of cytoskeletal function. In the second experiment, no difference in morphology was observed between oocytes retrieved from immature rats treated with either 4 or 40 IU PMSG. When compared with mature rats, changes attributable to cytoskeletal instability were observed in aged oocytes from immature rats treated with both doses of PMSG. This was concluded to be a manifestation of altered intrafollicular oocyte maturation as a result of the administration of exogenous gonadotrophin. In the third and fourth experiments, delayed fertilization and a significant reduction in fertilization rate were observed in superovulated, immature rats. The major cause of fertilization failure was determined to be intraoviductal oocyte aging. A significant increase in abnormal embryos was observed as a result of parthenogenetic activation of the aged oocytes. Abnormal, fertilized embryos retrieved from the superovulated group were concluded to be the manifestation of delayed fertilization. In conclusion, the major effect of a superovulatory dose of PMSG on oocyte fertilizability and embryonic development was intraoviductal oocyte aging and delayed fertilization. Changes attributed to altered intrafol1icular maturation were manifested during oocyte aging in immature rats treated with either the low or high dose of PMSG. / Medicine, Faculty of / Obstetrics and Gynaecology, Department of / Graduate