Two experiments were conducted on 272 young broiler cockerels to determine the effect of rye level, degree of saturation of added fat and supplementation with enzyme preparations containing xylanase and β-glucanase on the nutritional value of rye-containing diets fortified with 8.5 (Experiment 1) or 6% (Experiment 2) fat. In Experiment 1, increasing the rye level (0, 15, 30 and 45%) in the diet containing 11.5% crude mixed fat significantly depressed (P ≤ 0.001) organic matter retention (OMR), protein (APD) and fat (AFD) apparent digestibility. The metabolizable energy value (AME_N) of the 45% rye diet was 8.5% lower than predicted because of this depression. Supplementation of diets with an enzyme preparation significantly improved OMR, AFD, AME_N (P ≤ 0.001) and APD (P ≤ 0.01) in ryecontaining diets. The AME_N of the supplemented diet containing 45% rye was 1.7% higher than predicted. In Experiment 2, substitution o f unsaturated (soya oil) with more saturated fats (lard or tallow, or soya oil and tallow 1:1) in the diet containing 30% rye, caused a significant decrease in AFD (P ≤ 0.001), OMR and APD (P ≤ 0.05). The AME_N of the tallow-containing diet was 1.28 MJ/kg DM lower than o f the soya oil-containing diet. Cockerels fed the tallow-containing diet were lighter and used 13% more feed per gram of body weight gain (BWG). The degree of saturation of added fat did not affect the viscosity of intestinal digesta, but the viscosity of droppings was significantly higher in birds fed saturated fats. Enzyme supplementation significantly reduced (P≤0.001) both intestinal and excreta viscosity, OMR, AFD and AME_N. Cockerels fed enzyme-supplemented diets were on average 10% heavier and used 15% less feed per gram BWG. The greatest difference in the AME_N value (1.65 MJ/kg DM) was noted between supplemented and unsupplemented tallow-containing diets. The results of these experiments suggest that enzyme supplementation is necessary in particular if broiler diets contain both a high level of rye and saturated animal fat.