The pharmacodynamics of ethinylestradiol (EE2) were studied utilizing a specific radioimmunoassay to measure serum levels in subjects ingesting EE2 alone or in combination with various gestagens. Following the administration of a single oral dose of 20 micrograms of EE2 to three postmenopausal women, peak serum levels ranging from 29 to 58 pg/ml were reached within one hour. Levels fell below the sensitivity of the assay (20 pg/ml) 2, 5 and 8 hours after ingestion. In normal menstruating women, peak serum EE2 concentrations averaged 84 +/- 27 pg/ml after the ingestion of 35 micrograms EE2 and 0.5 mg norethindrone, 152 +/- 57 pg/ml after the ingestion of 50 micrograms mestranol and 1.0 norethindrone and 201 +/- 11 pg/ml after the ingestion of 50 micrograms EE2 and 0.5 mg norgestrel. After the ingestion of mestranol, peak serum levels of EE2 were less than and occurred later than peak levels obtained after the ingestion of the same dose of EE2.