Wistar rats (ten per group) were fed either an oat-free control diet or a dietary fibre-rich test diet containing 500 g oat-based products/kg for 6 weeks. The oat-based products, containing 4–128 g/kg resistant starch, 30–92 g/kg β-glucan and 122–304 g/kg total dietary fibre, were oat flour extrudate, flour/Novelose (commercial resistant starch) extrudate (80:20 w/w), oat bran, bran/Novelose extrudate (80:20 w/w) and autoclaved oat flour. Serum total cholesterol decreased in the groups fed flour, flour/Novelose and bran/Novelose (P<0·05). In most of the test groups, count numbers of bifidobacteria were higher (P<0·001) and of coliforms were lower (P<0·05). The mass of the caecum walls and contents was greater in groups fed Novelose- and bran-containing diets (P<0·005). In all the test groups, pH values were lower in the intestinal contents (P<0·001), and caecal concentrations of acetate (P<0·001), propionate (P<0·05), butyrate (P<0·005) and total SCFA (P<0·001) were higher. The lowest concentrations of steroids were found in rats fed the autoclaved flour. In the other test groups, more bile acids appeared in the caecal (P<0·001) and colonic contents (P<0·005), as well as in the faeces, at week 6 (P<0·001). The highest bile acid excretion was found after feeding bran-containing diets. In the intestinal contents of all the test groups, more primary bile acids (P<0·001) appeared than in the control group. The excretion of steroids increased within the experimental period. Using extrusion technology, dietary fibre-rich oat-based products, which have beneficial physiological effects in rats, can be produced. Oat flour and bran are excellent sources for the preparation of directly edible oat products. Their nutritional properties can be further improved by the addition of resistant starch.