The digestion of non-starch polysaccharides (NSP) was studied in rats given semi-purified diets containing 0-500 g raw peas (Pisum sativum)/kg. NSP were equally well digested at all inclusion levels with digestibilities for individual constituents ranging from 0.58 for xylans to 0.99 for arabinose-containing polymers with a total NSP digestibility of 0.79. Increasing the dietary pea inclusion rate increased the amount of substrate flowing to the large bowel (LB) and this was associated with marked increases in caecal tissue and contents masses, a reduction in caecal transit time from 0.88 to 0.43 d and a threefold increase in faecal bacterial output. Caecal pH fell as did the molar proportions of acetate, isobutyrate, isovalerate and valerate whilst butyrate increased when peas were included in the diet. Possible mechanisms for these fermentation end-product changes are discussed. Pea inclusion in the diet was associated with increased volatile fatty acid and 3-hydroxy butyrate concentrations in portal and heart blood. It was concluded that peas are a rich source of fermentable polysaccharides which produce a LB fermentation pattern of potential health benefit.