Six healthy subjects, 45-72 years old, received a 10-day feeding of 750 mg of two of the following bile acids: deoxycholate (DCA), chenodeoxycholate (CDCA), cholate (CA), hyodeoxycholate (HDCA), ursodeoxycholate (UDCA), and ursocholate (UCA). The urinary excretion of total bile acids was low during administration of lipophilic bile acids (DCA and CDCA), when serum levels show low postabsorption peaks. Instead, hydrophilic bile acids (UDCA and above all HDCA) were heavily excreted in the urine as sulphates and glucuronides, and serum levels reach high values. Only UCA, strongly hydrophilic, was predominantly excreted as unconjugated fractions. Thus, the physicochemical properties of bile acids (as measured by both the partition between octanol and water, and the water solubility) were factors that influenced the route of bile acid elimination from the body, whereas their conjugation was not always requested for urinary excretion.