Although it is now generally regarded that the origin of urinary trimethylamine (TMA) is via the action of intestinal microflora on precursors such as choline, little direct evidence exists. The normal production of urinary TMA was shown to be absent in germ-free mice and greatly reduced in antibiotic-pretreated animals. Cohabitation of germ-free mice with conventional animals restored their ability to excrete TMA. This study invokes a fundamental role for the intestinal microflora in the provision of TMA from precursors within the food.