We administered [6,7-3H]estradiol p.o. and [4-14C]estradiol i.v. simultaneously to 5 women 22-35 years of age. Fourteen blood samples were collected over 480 min, and all urine was collected for 96 h. The blood samples were analyzed for radioactivity as estradiol, estrone, estrone-sulfate and estradiol glucuronide. The urine samples were analyzed for radioactivity as the glucuronide and pH 1 hydrolyzable conjugates of estradiol, estrone, estriol, 16 alpha-hydroxy-estrone, 2-hydroxy-estrone, 2-hydroxy-estradiol, 2-methoxy-estradiol and 2-methoxy-estrone. The major circulating estrogen, after either estradiol, p.o. or i.v. administration, was estrone sulfate; approximately 50% of estradiol administered by either route being converted to and measured as estrone sulfate in the blood. Following oral administration about twice as much estradiol was converted to and measured as estradiol glucuronide in the blood as after i.v. administration. Of the estradiol administered p.o., only 10% was absorbed into the blood as estradiol the rest being metabolized prior to absorption. After estradiol, p.o., the major radioactive compounds in the urine were the glucuronides of estrone and estradiol, but after estradiol, i.v., the conjugates of estrone, estradiol and estriol were present to about the same extent as the conjugates of the 2-oxygenated compounds. Following p.o., considerable metabolism of estradiol administration occurs in the splanchnic tissue, much of it in the intestinal wall.