Excretion of metronidazole (MNZ) in the normal and in the diseased biliary tract was investigated in 58 patients after oral or intravenous administration of MNZ. After oral administration MNZ appeared rapidly in hepatic bile, and throughout the period of absorption and elimination almost identical concentrations of MNZ were found in serum and hepatic bile. After intravenous administration no significant differences were found between concentrations of MNZ in common duct bile and serum in the non-obstructed common duct; in common duct obstruction, concentrations of MNZ in common duct bile were 56--99 per cent of corresponding concentrations in serum. MNZ was concentrated in normal gallbladders. In patients with gallbladder stones and preserved function of the gallbladder and in patients with no function of the gallbladder but a patent cystic duct, no significant differences were found between concentrations of MNZ in gallbladder bile, common duct bile, and serum. In most gallbladders with the cystic duct blocked by a stone, no MNZ was found in gallbladder bile.