Specific RIAs requiring ether extraction only were established for estrone and 17 beta-estradiol both in plasma and in urine from the nonpregnant female. These assays were used to measure the renal clearance rates of estrone and of 17 beta-estradiol in eight ambulatory women in the follicular and in the luteal phases of the menstrual cycle. The mean (+/-SE) for the renal clearance rate of estrone was 0.71 +/- 0.058 ml/min in the follicular phase and 1.26 +/- 0.35 ml/min in the luteal phase. The mean (+/-SE) renal clearance rate of 17 beta-estradiol was 0.44 +/- 0.055 ml/min in the follicular phase and 0.29 +/- 0.043 ml/min in the luteal phase. There was no significant difference in the renal clearance rates of either estrone or of 17 beta-estradiol between the follicular and luteal phases of the cycle. The renal clearances of estrone and 17 beta-estradiol were highly correlated (r = 0.84; P less than 0.01). The renal clearance rate of estrone was significantly greater than that of 17 beta-estradiol in both phases of the cycle (P less than 0.01).