Changes in contents of hypothalamic luteinizing hormone-releasing homone (LH-RH) were studied in midpregnant rats. The hypothalamus was dissected out on days 7, 9, 12 and 15 of pregnancy. HL-RH activity in the extrancts was determined by a competitive radioimmunoassay method. The mean LH-RH content of the hypothalamus was the lowest on day 9, and gradually increased until day 15 (p<0.05). Midpregnancy surge of the serum estradiol levels is considered to inhibit through a negative feed-back the production of LH-RH in the hypothalamus.