Luteinizing hormone-releasing hormone (LHRH) degrading activiy may be of physiological significance as a mechanism capable of partial regulation of hypothalamic LHRH release as well as LHRH levels at the gonadotroph. The possibility of cyclic fluctuations in LHRH-degrading activity was investigated in female rat hypothalami and pituitaries. These tissues were collected at selected time points during the 4-day estrous cycle, homogenized, and centrifuged at 100,000 g. Supernatants were incubated with synthetic LHRH, the reactions terminated, and the decapeptide and its products separated by high-performance liquid chromatography. Degradation of LHRH incubated with active cytosol was estimated by comparison of integrated LHRH peak area with that from incubations with heat-inactivated cytosol. Hypothalamic LHRH degradation was depressed during the latter hours of diestrus 2, a time during which the LHRH content in the hypothalamus has been reported to be increasing. From diestrus 24.00 h to proestrus 15.00 h, there was a significant increase in degrading activity. This was then followed by a decline from 15.00 to 18.00 h proestrus; at the time of the LH surge, the activity had not undergone significant increase in comparison to 18.00 h. Pituitary LHRH degradation was significantly increased during the 6-hour period preceding the surge, but was significantly depressed at the surge. The hypothalamic reduction in activity associated with diestrus 2 as well as the hypothalamic and pituitary reductions associated with proestrus may represent a permissive effect allowing increased LHRH accumulation in the hypothalamus and its prolonged action in the pituitary. These cyclic variations in net LHRH degradation may arise from modulation of one or several peptidases known to degrade the hormone and give further evidence suggesting that the ovulatory cycle may in part be strongly influenced by LHRH-degrading activity.