Intestinal intubation was carried out in 21 subjects: 9 with end-stage renal failure, 2 with early renal insufficiency, 7 untreated patients with blind-loop syndrome, and 3 normal volunteers. All 9 patients with uraemia had significantly raised duodenal dimethylamine (D.M.A.) concentrations compared with the other groups tested. Alteration of the intestinal bacterial flora with antibiotics markedly reduced serum D.M.A. and trimethylamine concentrations and modified the symptoms. Potentially toxic metabolites in the small bowel might have significant nutritional and toxic sequelae in uraemia and these findings suggest that current therapeutic approaches to the treatment of end-stage kidney disease should be re-examined in relation to bowel flora.