Using both pulse injections and constant infusions of 3-H-mestranol (3H-ME) (1) and 3-H-ethinyl estradiol (3H-EE) we have studied the metabolism of these compounds in non-users and users of oral contraceptives. Following pulse injection of 3-H-ME the disappearance of radioactivity could be described as a function which was the sum of two exponentials. Studied by both types of administration there was no difference in the metabolism of 3H-ME in the two groups; the overall mean plus or minus SE metabolic clearance rate (MCRM) was 690 plus or minus 45 1/day/m2, the mean ratio of the concentrations of radioactivity as EE following administration of ME (CRM-BB,E) was 0.23 plus or minus 0.02 and the mean (p)M,EBB (fraction of administered ME measured in blood as EE) was 0.19 (95% confidence limits equals 0.15 - 0.23). Following pulse injection of 3-H-EE the disappearance of radioactivity was best described as a function which is the sum of three exponentials. Results from both types of administration revealed no difference in the metabolism of 3H-EE between non-users. The overall mean plus or minus SE MCREE was 630 plus or minus 30 1/day/M2. The MCREE is significantly (0.02 greater than P greater than 0.01) less than the mean MCR for estradiol reported previously, in both non-users and users of oral contraceptives. The use of oral contraceptives containing estrogens and progestins does not appear to influence the metabolism of the estrogen used. Approximately 20% of mestranol is converted to and appears in the blood as ethinyl estradiol.