Cycling female rats were sacrificed at various times during the 4-day oestrous cycle. LH-RH in the medial basal hypothalamus (MBH) and serum LH-RH, LH, oestradiol (Oe2), and progesterone were analyzed by radioimmunoassays. The MBH LH-RH content was lowest at 19.55-20.15 h during pro-oestrus but increased gradually through oestrus and dioestrus I to significantly higher values at noon of dioestrus II, and then decreased precipitously at 18.00 h. Although serum LH levels remained basal from midnight of pro-oestrus through dioestrus II, serum LH-RH levels were significantly elevated at 12.00 and 21.00 h on oestrus (vs. the midnight pro-oestrus levels) and declined between 15.00 and 22.00 h of dioestrus I. In conjunction with high Oe2 titres, LH-RH in the MBH and serum declined initially during pro-oestrus between 14.07-14.45 h and then increased abruptly between 14.55-15.55 h; the pre-ovulatory rise in serum LH was observed from 16.05 h onwards. LH-RH activity in the MBH and serum receded gradually to the early afternoon levels by 19.55-24.00 h while the peak in serum LH was observed at 17.05-18.05 h. These studies suggested that the hypersecretion of the MBH LH-RH (synthesis + release) may be responsible for the pre-ovulatory discharge of LH.