Effects of short and long term administrations of an oral contraceptive preparation on the pituitary responses to the synthetic LH-RH were compared in the present experiment. Five normal cycling women were used as controls in the late proliferative phase. Five healty volunteers taking an oral contraceptive agent (1.0 mg norethindrone with 0.05 mg mestranol) for less than 10 cycles were served as short term group and other 4 women taking the same preparation for more than 40 cycles were studied as long term group. Volunteers in each group were kept fasting overnight and 5 ml of blood was drawn at 9 to 10 am on the day of experiment. Ten minutes after drawing blood, 200 mug synthetic LH-RH was injected subcutaneously in the late proliferative phase in the normal cycling women. According to the same procedure, women taking pills were administered LH-RH on the 7th to 12th day after the beginning of taking the first tablet in each cyle. Three ml of blood was taken at 15, 30, 60, 120 minutes and 24 hours, and serum LH and Fsh were determined by the double antibody radioimmunoassay. The 2nd IRP-HMG was used as the standard materials and expressed as mIU/ml of serum. Mean baseline serum LH and FSH concentrations were not suppressed in a short term administration of norethindrone-mestranol combination. Fifteen minutes after the subcutaneous injection of LH-RH, mean LH level was significantly elevated, and thereafter the level was not significantly changed as compared with that seen in the late proliferative phase of the cycle. Concerning the FSH response to LH-RH, a short term administration did not induce a significant rise. ...