Intestinal absorption, biliary excretion and metabolism of a calcium gallstone dissolving agent, [11, 12-³H] ursodeoxycholyl-N-carboxymethylglycine (UDC-CMG) and its monoethyl, diethyl and dipivaloyloxyethyl esters (UDC-CMG-Et, UDC-CMG-Et₂ and UDC-CMG-PV₂) were studied in bile duct cannulated rats. Biliary recovery of [³H]-labeled UDC-CMG, UDC-CMG-Et and UDC-CMG-Et₂ after intraduodenal administration were 65%, 80%, 98%, respectively. Radio-thin layer chromatography analysis of the bile revealed that UDC-CMG didn't undergo any biotransformation during administration and excretion. About 80% and 20% of radioactivity recovered in the bile was identified as UDC-CMG-Et and UDC-CMG, respectively, after intraduodenal administrations of [³H] UDC-CMG-Et₂ and [³H] UDC-CMG-Et. The administered intact UDC-CMG-Et₂ was not found in the bile. Intraduodenally administered [³H] UDC-CMG-PV₂ was rapidly recovered in the bile. The total recovery rate was 78% within a 24 h period. More than 80% of the radioactivity recovered in the bile was found as UDC-CMG. Lesser amounts of the monopivaloyloxyethyl ester of UDC-CMG were also found, but intact UDC-CMG-PV₂ was not detected in the bile as in the case of UDC-CMG-Et₂. Among the esters of UDC-CMG investigated in the present studies, only UDC-CMG-PV₂ was excreted in the bile mainly as the perhydrolyzed form, UDC-CMG. These results suggest the usefulness of UDC-CMG-PV₂ as the pro-drug in calcium gallstone dissolution therapy.