Levels of progesterone, estradiol, LH, and FSH were measured in daily serum samples obtained from 4 subjects during a control cycle and during the first and fourth menstrual cycles after insertion of an intrauterine device (IUD). In addition, progesterone and estradiol were measured in serum samples obtained from 6 women 3, 4, or 5 months after IUD insertion, and from 6 women more than 1 year after IUD insertion. These measurements were compared to the data obtained from study of a large group of normal cycles. The results indicated that the IUD does not influence follicular maturation, time of ovulation, or corpus luteum function. The IUD did exert a local effect on the endometrium. causing the onset of menses to take place when steroid levels were higher than in control cycles.