Superovulation with exogenous gonadotropins has been used widely to study the reproductive events in animals and in vitro fertilization and embryo transfer in human beings. However, details of the mechanism of superovulation are not yet clearly understood. The present study was conducted to study the mechanism of superovulation with purified human FSH and LH preparations. Cyclic hamsters received continuous infusion of purified FSH and LH for 4 days, from estrus to proestrus. The number of ova shed was significantly increased by 5 iu FSH/minipump infusion, and further increased by 5 iu FSH/minipump plus 5 iu LH/minipump group. On the other hand, the number of atretic follicles was decreased significantly (P < 0.01) by the combined infusion of FSH and LH, but not by FSH alone. Among these experimental groups, there was no significant difference in embryonic development which was assessed by the counting of the number of 4-cells and more developed embryos on day 3 of pregnancy. These data indicated the different roles of FSH and LH in the superovulation process, because FSH stimulated some small follicles to grow and LH, in the presence of FSH, rescued the follicles to undergo atresia.