The mechanism of action of levonorgestrel (LNG) and mifepristone (MIF) in emergency contraception (EC), is still not fully known. The purpose of this study was to evaluate the effect of preovulatory treatment with LNG and MIF on luteal function in more detail. Two days prior to ovulation (day -2; assessed by ultrasound), we administered LNG (0.75 mg twice, 12 h apart) or MIF (10 mg, single dose) to seven women in different cycles. Follicle development was followed by ultrasound. Urinary estrone glucuronide (E1), pregnanediol glucuronide (P4) and luteinizing hormone (LH) were analyzed by enzyme immunoassays daily starting with day -2 for the rest of the menstrual cycle, along with urinary creatinine (C). The treatment caused either a delay or an inhibition of the LH peak in all subjects. A significant delay in P4 levels and an initial suppression of E1 levels were also noted. The development of the leading follicle was either arrested or continued without signs of rupture. This study indicates that, when used for EC, LNG or MIF administered prior to ovulation acts through an impaired ovulatory process and luteal function.