Twelve non-obese patients with radiolucent gallstones were fed on a standard diet. After 10 days (period A), six patients received 15 mg/kg/day of ursodeoxycholic acid (UDCA) (group I) and the other six (group II) the same dose of chenodeoxycholic acid (CDCA) for 15 days (period B). An intravenous injection of 20 micro Ci of 14C-UDCA and of 14C-CDCA was given on the 11th day of period B to the patients of group I and II respectively. Stools were collected at the end of period A and B and one bile sample was collected on the 12th day of period B. The faecal bile acid loss was higher during chenotherapy (36.12 mumol/kg/day) than during ursotherapy (23.94 mumol/kg/day), as was the proportion of lithocholic acid (73% vs 43%) in the faeces. Decay constant rate of faecal radioactivity was 0.365 day-1 in group I and 0.642 in group II. The results indicate that faecal bile acid excretion and turnover rate are greater during CDCA than UDCA, while UDCA increases the bile acid pool size to an even greater extent than does CDCA (150.2 vs 94.9 mumol/kg). This is probably because the former is more slowly degraded to poorly reabsorbable compounds. In fact, the bile saturation index was 0.66 in group I and 1.05 in group II, even though biliary CDCA in the latter had risen to 69.6%.