The aim was to compare the effects of hormone treatments on the ovari- an follicular development and in vitro embryo development between 5-week and 13- week old lambs. FSH and pregnant mare serum gonadotropin （PMSG） treatment were used to compare the number of 2-8 mm follicles in the ovary and the in vitro embryo development. The results showed that： the 5-week old lambs had larger fol- licular numbers and collected oocytes numbers （58.6±1.9, 47.4±4.2） than the 13- week old lambs （14.4±4.1, 11.6±1.7）, and the difference was significant （P〈0.05）. However, there was no significant difference in the cleavage rate （60.0±1.9%, 61.6± 5.2%） and blastocyst rate （27.4±2.1%, 28.2±3.7%） of the fertilized oocytes between the 5-week and 13-week old lambs.
