Abstract Labeled estriol (E 3 ) was injected i.v. into intact guinea-pigs and animals with biliary fistulas. It was shown that biliary excretion constitutes a major route for the conjugates of E 3 , probably followed by a significant enterohepatic circulation. The conjugation pattern of E 3 revealed the preponderant amount of the steroid to be excreted as a 3-glucosiduronate, both in the urine and bile. In contrast, the 3,16α-disulfate of E 3 was found in significant amount only in the bile. Unconjugated E 3 and its sulfate constituted a minor percentage of the radioactivity found both in bile and urine. The E 3 nucleus remained intact during the conjugation processes in the guinea-pig. The kidney was shown to glucoronidate E 3 in vitro to some extent, but to a much lower degree than observed with kidneys of other species; sulfation of E 3 by the guinea-pig kidney was of an even much lower magnitude. Thus, we interpret the results to indicate that in guinea-pigs, the liver plays a paramount role in the conjugation of E 3 followed by biliary excretion of most of the conjugates and subsequent entero-hepatic circulation. The role of the kidney in the conjugation of E 3 in the guinea pig appears to be secondary to that of the liver.