Recent evidence indicates that estradiol (E) is involved in the regulation of LHRH mRNA in hypothalamus as well as LH mRNA in anterior pituitary. The relationship between LHRH and LH gene expression remains, however, unknown. The present study aims to codetermine alterations in LHRH mRNA in the hypothalamus and α and LHB in the anterior pituitary in response to E treatment. Specifically the followings were tested: 1) the dose effect of E, 2) the time-course of E following ovariectomy and 3) the effect of E antagonist, LY117018 on the E-induced changes in LHRH and LH mRNA levels. Adult female rats were ovariectomized and three weeks later, either E or E + LY117018 was administered sc into ovariectomized rats. LHRH, α and LHB mRNA levels were determined by RNA blot hybridization assay. A single injection of E (0.1-1 ㎍) resulted in a dose-related decrease in LHRH mRNA as well as α and LHB mRNA as compared to the levels observed following ovariectomy, although E (100 ㎍) induced a slight increase in LHRH mRNA. E (10 ㎍) produced a significant decrease in LHRH and LH mRNA 48 hr after E treatment. The concomitant administration of E and LY117018 for 2 days restored the E-induced decreases in both LHRH mRNA and LH mRNA. These results suggest that E regulates LHRH mRNA and LH mRNA in a well-coordinated manner.