Beeturia is the excretion of red beetroot pigment (betalaine) in urine and faeces. It occurs in about 14% of humans. Betalaine is a redox indicator whose colour is protected by reducing agents. We investigated pigment-decolourizing systems in the intestinal tracts of beeturic and non-beeturic subjects. Betalaine was decolourized by hydrochloric acid, ferric ions and colonic bacteria preparations, but not by pancreatic or mucosal enzymes. In animals, oral betalaine did not produce beeturia, but injection of betalaine into the peritoneum did. Oral betalaine and 1 g oxalic acid produced beeturia in non-beeturic normal humans, but passed into ileostomies without beeturia. Thus, beeturia results from colonic absorption of betalaine. Oxalic acid preserves the red colour to the colon, otherwise it is decolourized in non-beeturic individuals by nonenzymic processes in the stomach and colon.