Renal clearance of estriol (E3), E3-3-sulfate (E3S), E3-16-glucuronide (E316G), E3-3-glucuronide (E33G), and E3-3-sulfate-16-glucuronide (E3SG) has been measured in 13 normal and 17 abnormal pregnancies between the 33rd and 40th weeks of gestation. The methodology involved a chromatographic separation on a celite column in the presence of tritiated tracers, enzymic hydrolysis of the conjugates, and measurement of the E3 moiety by a specific RIA. Preeclampsia (eight patients) was characterized by a significant decrease of renal clearance for all conjugates except E33G. In contrast to that pattern, a patient with cholestatic icterus had normal clearances except for E33G, which was reduced to less than 15% of its normal mean value. Administration of ampicillin to a pregnant woman induced an important decrease of all E3 clearances, especially for E33G which decreased to nearly 1% of its initial value; normal clearances were resumed within a week after the end of the treatment. In a small number of complicated pregnancies, E3 clearances were normal. A significant correlation between the renal clearance of E316G and that of urea and creatinine has been demonstrated.