Studies on estriol metabolism and production were carried out in normal reproductive-aged and post-menopausal women using pulse injections and constant infusions of radiolabeled estriol, estrone and estradiol. The circulating levels were measured by radioimmunoassay. Following an intravenous pulse of [3H]estriol, the disappearance of radioactivity as estriol could be described as a function which was sum of two exponentials with t 1/2's of 3.6 and 64 min. The initial volume of distribution was 201. Using the constant infusion technique the metabolic clearance rate (MCR) of estriol was 2100 1/day in the follicular phase of the cycle and similar in the luteal phase. In postmenopausal women the MCR was 1890 1/day. The circulating levels of estriol were 7 and 11 pg/ml in the follicular and luteal phases respectively and 6 pg/ml in post-menopausal women. The production rates of estriol were 14 and 23 micrograms/day in the follicular and luteal phases of the cycle and 11 micrograms/day in post-menopausal women. In many of the women infused with radiolabeled estrone or estradiol, no radioactivity could be identified in the blood as estriol. The maximal conversion of estrone and estradiol was less than 0.4%. Estriol circulates at low but relatively steady levels in the blood. In some women estriol appears to be secreted by the ovary, especially in the luteal phase.