The purpose of this experiment was to investigate the effects of synthetic gonadotropin releasing hormone (GnRH) administration on serum LH level and uterine weight in mature female rats. The rats were given a intraperitoneal injection of synthetic GnRH at 1:00 PM of proestrus, and were sacrificed immediately after administration, at 2:00, 3:00, 4:00, 5:00, 6:00, 7:00, 8:00 PM to investigate the changes of serum LH level and uterine weight. 1. In control group, the serum LH level increased from 4.35±0.84 mIU/㎖ at 1:00 PM to a preovulatory LH surge about 11.05±1.4 mIu/㎖ at 5:00 PM and then decreased gradually. The serum LH level in group treated with synthetic GnRH remarkably increased from 4.20±0.30 mIU/㎖ at 1:00 PM to a peak level, 13.17±0.17 mIU/㎖ at 2:00 PM, 1 hour after administration. The time required up to the preovulatory LH surge in treated group was shortened about 3 hours as compared to that of control group. 2. The weights of uterus in group treated with synthetic GnRH were slightly heavier than those of control group at 1, 2 and 3 hour after administration, but there were no significant differences between the control and treated group.