The relationship between the hypothalamo-pituitary-gonadal (HPG) axis and the hypothalamo-pituitary-adrenal (HPA) axis has been well documented in the rat. In most cases, a negative coupling was observed and an inhibitory effect of the HPA axis upon the HPG was shown. In the female rat, a marked circadian rhythm of corticosterone plasma values is observed during each day of the estrous cycle, with maximal values around 08:00 p.m. The preovulatory luteinizing hormone (LH) surge also occurs at 08:00 p.m. on the day of proestrus. Here we measured circadian variations of plasma cortisol in humans in relation with the time of initiation of the preovulatory LH surge. Blood samples were taken at 08:00 a.m., 12:00 a.m., 04:00 p.m., 08:00 p.m., 12:00 p.m., and 04:00 a.m. from 19 subjects for 4 consecutive days, once 17β-estradiol (E₂) values reached 125 pg/ml (days 7–10 of the menstrual cycle). Serum E₂ and LH determinations were performed by microparticle enzyme immunoassays. Serum progesterone and plasma cortisol determinations were made using RIA methods. For plasma cortisol values, a marked circadian rhythm, with 2- to 3-fold higher values during the morning than during the afternoon, was almost identical before, during and after the LH surge. However, values were generally higher during the follicular phase than during the luteal phase. Maximum cortisol values occurred between 04:00 and 08:00 a.m. and minimal cortisol values between 04:00 and 08:00 p.m. Initiation of the LH surge (50% over the mean of previous values) occurred at 04:00 a.m. (20% of the cases) or at 08:00 a.m. (80% of the cases). There was a strong coupling between the onset of the surge and the acrophase of the cortisol circadian rhythm: maximal cortisol plasma values were seen at 04:00 a.m. when the LH preovulatory surge started at 04:00 a.m. and 08:00 a.m. when it started at 08:00 a.m. The present results show that the positive coupling documented in the female rat between the HPA and the HPG axis at the time of preovulatory LH surge is also present during the menstrual cycle in the human.