Because we had observed that smoking has a pronounced effect on serum progesterone levels, we reinvestigated in healthy nonsmokers the relative progesterone levels of men and follicular-phase women. Each of eight women had multiple measurements of serum progesterone during the follicular phase of a menstrual cycle (10 days through 3 days prior to the luteinizing hormone peak of that cycle), and the average of those values was taken to represent the basal progesterone level for that woman. Seven men had blood samples drawn at 20-minute intervals between 6:00 and 9:00 AM, through an indwelling venous catheter, and the average of those values was taken. The mean follicular-phase serum progesterone level in the women was 21.4 +/- 5.4 ng/dl and the mean level in the men was 18.1 +/- 3.1 ng/dl. The difference was not statistically significant. In view of this finding, we conclude that there is essentially no ovarian secretion of progesterone during the follicular phase of the menstrual cycle.