The aim of the present experiment was to study the effect of an increasing level of an enzyme preparation on intestinal viscosity, ileal digestibility and performance of broiler chickens fed a diet containing wheat (63%) and rye (9.35%) in the period from 0-21 days of age. Four diets, containing 11.98 MJ ME/kg feed and 19.52% protein, were included in the experiment: Control (basal diet), L, M, and H (basal diet with 100, 200 and 300 mg Bio-Feed Wheat CT/kg, respectively). Feed conversion in chickens fed the diet with the medium dose of enzyme was significantly (P<0.002) better compared with the control group and the chickens fed the diet with the lowest dose of enzyme by 5.0 and 3.3%, respectively. Weight gain of chickens fed diet M was improved by 5.3%, and was significantly (P<0.0008) different from all the other groups. Enzyme addition reduced viscosity in jejunum (P<0.007) and ileum (P<0.05), diet L being the exception. A significant (P<0.001) improvement in ileal protein digestibility was found in the chickens fed diet M, the increase being 12.4% compared with the control, confirming the results found on performance. Enzyme addition to the other groups increased the ileal protein digestibility on average by 3.5%, but not significantly different from the control. A pronounced effect of enzyme supplementation was found on the ileal fat digestibility of chickens fed diet M, the improvement (P<0.0007) being as high as 17.0%. In addition, the ileal digestibility of organic matter in chickens fed diet M was improved by 8.9% compared with the control group (P<0.02).