The fasting peripheral serum concentrations of cholic acid, chenodeoxycholic acid, deoxycholic acid and ursodeoxycholic acid were determined in a total of 32 patients with uraemia. Eighteen of the patients suffered from diarrhoea. A highly accurate and specific mass-fragmentographic technique with high sensitivity was used to measure the concentrations of the individual bile acids. The total amount of bile acids was normal in the two groups of patients. Patients with diarrhoea had 50% lower concentrations of deoxycholic acid than a corresponding group of healthy subjects. It is suggested that the low level of deoxycholic acid may contribute to diarrhoea in these patients.