Germ-free and conventional rats were fed a semi-synthetic diet including 10% wheat bran for 1 month. They were compared to similar rats fed the same diet without bran. Transit of polyethylene glycol (PEG) 4000 was studied in giving the marker either in saline or in the diet. The presence of a bacterial flora was found to accelerate small intestine transit and total transit. The ingestion of solid food slowed gastric emptying but had little effect on small intestine transit. Wheat bran slowed gastric emptying of PEG in most of the experimental groups. It had no significant effect on small intestine transit except in germ-free rats having received the marker in solid food. It accelerated total transit in germ-free and conventional rats. The results suggest that bran mainly exerts its action in the hindgut. The role of microflora in this action of bran is discussed.