The concentrations of the free (F), glucosiduronate (G), sulfoglucosiduronate (S:G) and sulfate (S) forms of estriol (E3), estrone (E1) and estradiol (E2) in the plasma and urine of 5 normally pregnant women have been measured before and during parturition. Of the total plasma E1 + E2 + E3, 25–80% was E3 with 15–60% of this accountable to G, 15–40% to S and 7–42% to S:G, these being the major forms of circulating E3. Five to 60% of the total was E1, with 50–90% of this accountable to S, the major circulating form of E1. Five to 15% of the total was E2 with 70–95% of this accountable to F, the major circulating form of E2. These wide ranges of distribution were due to marked changes in plasma concentrations which followed consistent patterns for each constituent before and during labor. Just before labor there was a decrease in the concentration of all of the major forms. During labor the amount of circulating E3-G and E3-S:G continued to decrease while E3-S, E1-S and E2-F increased. Plasma and urinary levels did not run parallel. Of the total urinary E1 + E2 + E3, 75–97% was E3 with 85–97% of this accountable to G and 2–13% to S:G + S; 2–21% of the total was E1 with 1.5–7.5% of this accountable to S; and 1–4% of the total was E2 with less than 2% of this accountable to F. Thus there was a considerably higher concentration of E3-G in the urine than in the plasma; and 90–98% of E1 and E2 were excreted as G, whereas only trace amounts of these 2 hormones circulated as G. Urine to plasma ratios indicated that the glucosiduronates of all estrogens were secreted by the kidney tubule while the sulfurylated and free forms were reabsorbed. Both renal mechanisms were accelerated before and during labor. They failed to fully explain the preparturition decreases in plasma concentrations.