1. Seventeen pigs fitted with single re-entrant cannulas in either the duodenum (posterior to the point of entry of the bile and pancreatic ducts), the mid-jejunum, or the terminal ileum, and a further twenty-four non-cannulated pigs were used.
2. Three diets were used: these contained barley, fine wheat offal, white fish meal, minerals and vitamins (diet BWF); starch, sucrose, maize oil, cellulose, minerals, vitamins and either groundnut meal (diet SSG) or casein (diet SSC).
3. The quantities of total lipid (TL), acid-detergent fibre (ADF) and volatile fatty acids (VFA) passing through the re-entrant cannulas and excreted in the faeces in 24 h were measured. These were used to determine the net absorption (or synthesis or secretion) in the different regions of the intestine.
4. There was substantial secretion of TL into the proximal small intestine and major absorption from the distal portion for all three diets. In the large intestine there was little or no further net absorption for diets SSG and SSC, but significant synthesis or secretion for diet BWF.
5. Fibre digestion occurred in both the small and large intestines and the apparent absorption of ADF in the different regions studied was influenced by diet.
6. There were detectable amounts of VFA at all sites but only at the ileum and in the faeces could they be accurately measured. Diet did not affect the molar proportions of acetate, propionate and butyrate in ileal digesta, but did have an effect on these VFA in the faeces.