1. Transit time of digesta, faecai volatile fatty acids (VFA) excretion and faecal output were measured in four pigs (initially of 90 kg live weight) fitted with chronic catheters inserted into the caecum.
2. Each pig was given a basal milk diet (20 g/kg live weight per d) for 30 d and then received successively four further treatments: the basal diet with bran (100 g/d), the bran-supplemented diet with a daily administration of neomycin (100 mg/kg orally and 100 mg/kg intracaecally), the basal diet with continuous intracaecal infusion of either saline (9 g sodium chloride/1) or with VFA solution (100 mM/h) at the same rate.
3. The mean retention time of the polyvinyl chloride marker was reduced from 98·6 h on the milk diet to 64·3 h on the milk + bran diet. This transit time was notsignificantly modified by neomycin treatment.
4. Daily faecal excretion of VFA was significantly affected by the diet: the additionof bran induced a 167% increase from the milk diet; neomycin treatment reduced VFA excretion with the bran-supplemented diet from 11·3 mM/d to 6·3 mM/d whereas during VFA infusion, excretion levels were twice those of the basal diet.
5. Infusion of VFA solution on the milk diet induced an 11% increase in transit time,without any change in faecal output and dry matter.
6. In conclusion, it is suggested that the decrease in transit time associated with bran supplementation is mediated by direct mechanical factors rather than fermentation products, including VFA.