Plasma levels of 2-hydroxyestradiol (2-OHE2) were measured using a new RIA procedure. Values were below the detection limit of the assay (less than 10 pg/ml), except in the third trimester of pregnancy, when they rose to approximately 15 pg/ml. The infusion of 130 microgram/h purified 2-OHE2 elevated its plasma concentration to 155 pg/ml, consistent with a plasma MCR (MCRp) of approximately 20,000 liters/day. The infusion of [3H] 2-OHE2 to equilibrium and chromatographic separation of the extracted plasma metabolites yielded an MCRp of about 13,000 liters/day; the major plasma metabolite comigrated with 2-methoxyestradiol, and [3H] xi-methoxyestrone was also formed. The MCRp, of 2-OHE2 is approximately half that of 2-hydroxyestrone (2-OHE1), but much higher than those of other steroids. As is true for 2-OHE1, the clearance of 2-OHE2 must occur primarily in the blood compartment. Together, the measured MCRp values and estrogen receptor affinities of 2-OHE2 and 2-OHE1 predict a relative potency for effects upon gonadotropin secretion which is close to that observed in vivo.