The objectives of this study were to determine whether intestinal viscosity caused by mixed linked barley beta-glucan depresses ileal nutrient digestibility and digestive enzyme activities and to determine the interaction of intestinal viscosity, digestive enzyme activities and ileal nutrient digestibility in different ages of poultry. In Experiments 1 and 2, 1-d-old broiler chicks and 1-y-old cocks, respectively, were fed diets with 60% corn, low and high viscosity barley with or without beta-glucanase, for 3 wk. A 3 x 2 factorial design was used. Comparisons were made only within the same age group. In Experiment 3, 1-d-old broiler chicks were fed high viscosity barley with and without beta-glucanase to measure fecal nutrient and ileal and fecal amino acid digestibility. Broiler chicks fed barley ate less and gained less weight than those fed corn; added beta-glucanase resulted in increases in both food consumption and weight gain for the barley-fed chicks (P < 0.05). Relative pancreas weight was higher (P < 0.05) in chicks fed barley than in those fed corn, and lower with beta-glucanase (P < 0.05). Digesta from barley-fed birds had the highest viscosities, which were decreased (P < 0.05) by beta-glucanase. Amylase and lipase were lower in broiler chicks fed high viscosity barley compared with corn (P < 0.05), and beta-glucanase increased both activities and that of trypsin as well (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)