The peripheral plasma concentrations of progesterone and 5alpha-pregnane-3,20-dione were measured daily throughout the ovarian cycle of five presumptively ovulatory women. The concentrations of progesterone and 5alpha-pregnane-3,20-dione during the follicular phase were 0.11 +/- 0.02 ng/ml (mean +/- SE) and 0.16 +/- 0.01 ng/ml, respectively. In four of these women the concentrations of both progesterone and 5alpha-pregnane-3,20-dione began to rise immediately after the luteinizing hormone (LH) peak and reached maximal concentrations five to eight days later (progesterone: 8.6 to 19.9 ng/ml; 5alpha-pregnane-3,20-dione: 1.4 to 2.8 ng/ml). The luteal phases in these women, measured from the day of the LH peak to the onset of the next menses, varied from 13 to 18 days. In the other woman with a late LH surge (day 27 of the cycle) the peak concentrations of progesterone and 5alpha-pregnane-3,20-dione were reached four days after the LH peak (8.9 ng/ml and 0.8 ng/ml, respectively). The luteal phase in this woman was 11 days. From the results of this study, it appears that the concentrations of 5alpha-pregnane-3,20-dione in peripheral blood during the luteal phase are proportional to the availability of progesterone as substrate for the 5alpha-reductase enzyme system.