These studies were designed to investigate the differences in blood plasma levels of ethynylestradiol (EE2) in women who developed hypertension while taking combined estrogen and progesterone oral contraceptives (OCs) and in normotensive OC users. Blood samples were collected in heparinized tubes 10 hours after OC ingestion, the plasma was separated, and EE2 was measured by radioimmunoassay. The results showed significantly higher plasma levels of EE2 in the hypertensive OC users as compared with the levels in normotensive OC users (P less than 0.01). In another study, blood samples from hypertensive and normotensive OC users were obtained for 3 consecutive days at fixed intervals following OC ingestion, and plasma levels of EE2 were measured. The results showed consistently higher EE2 blood levels during this 3-day period in the hypertensive subjects (P less than 0.01). It is postulated that the higher blood levels of EE2 in hypertensive OC users result from either decreased metabolism or excretion of synthetic estrogens.