Changes in the viscosity of intestinal contents and growth performance of broiler chickens fed oat-based diets, as influenced by oat cultivar (dehulled or naked oats cv. Akt or Polar) and dietary level (150 or 300 and 250 or 500 g/kg, in starter and grower diets, respectively) and enzyme supplementation were investigated. The relative intestinal viscosity was determined at 21 and 42 days of age. The dietary level of oats, their cultivars and enzyme supplementation of diets had a significant influence on body weight gains and feed conversion in broiler chickens. The enzyme supplement significantly (P≤0.01) decreased intestinal viscosity and improved performance parameters in chickens.