The bioavailability of 17 beta-estradiol valerate, free 17 beta-estradiol and Premarin was studied in 24 subjects suffering from secondary amenorrhea. 17 beta-estradiol and estrone were determined by specific radioimmunoassay systems. All estrogens were administered as single oral doses. The amounts applied were 2 and 10 mg of 17 beta-estradiol valerate or equivalent amounts of the free estradiol and Premarin, respectively. No elevations of 17 beta-estradiol serum levels were recorded at the lower dosages, whereas the increases of estrone serum levels were approximately tenfold over basal levels. Similar data were recorded in a group of 3 women with normal cycles after the ingestion of 2 mg of 17 beta-estradiol valerate. All subjects responded to the higher doses of estrogens with a moderate increase of 17 beta-estradiol serum levels, but with much higher elevation of estrone levels. These data suggest a rapid conversion of 17 beta-estradiol into estrone. The biological activity of 2 mg 17 beta-estradiol valerate is discussed on grounds of a possible metabolism of estrone sulfate at the target tissues.