A 5:3:2 mixture of rape seed oil, poultry fat, and beef tallow was saturated with oxygen and heated at 70-80°C for 48 to 96 h to obtain batches of fat having peroxide values of 5.1, 50.0, 100.5, and 208.6 meq O₂/kg and respective anisidine values of 2.6, 12.1, 22.2, and 95.9. Six casein diets containing 10% oxidized fat with peroxide values of 5, 40, 80, 120, 160 or 200 meq O₂/kg were prepared and fed for 8 weeks each to 12 individually housed rats with an initial body weight of 58.5 g. Body weight gain, feed intake and utilisation, PER, protein and fat digestibilities, VFA content in the caecum of two groups of rats, and β-glucuronidase activity in the caecal digesta, and phosphatase activity in the mucosa of the small intestine were measured. The degree to which fat was oxidized did not affect feed intake, body weight gain or protein digestibility. Feed utilisation was, however, lower in the groups fed diets with peroxide values of 160 and 200 meq O₂/kg. Fat digestibility was lower only with the highest degree of oxidation. Phosphatase activity decreased while that of glucuronidase rose as the degree of fat oxidation increased. The VFA concentration in the digesta of the caecum of rats fed the diet with the most oxidized fat was significantly lower than in the control group, which points to the unfavourable effect of oxidized fat on bacterial activity in the large intestine.