The aim of the study was to investigate the effect of digesta viscosity on the development of the digestive tract, motility of the small intestine and digestion of nutrients in the early stages of life of broiler chickens. At the age of 1 week sixty-six broiler Hybro females were allocated to 3 groups, 22 per group. The birds were caged individually and fed ad libitum a control, wheat-based diet supplemented with xylanase (1g/kg), or rye-based diets: R, non-supplemented or RE, supplemented with xylanase (3g/kg). Diets were cold pelleted. Body weight and feed intake were registered in weekly intervals. In the fourth week of life apparent fat and protein digestibility and metabolizable energy value of the diet were estimated in a balance study on 8 birds from each group. In the fifth week of life all birds were sacrificed and the extent of stomach distention was scored from 1 (normal) to 5 (most distended). Duodenum and jejunum fragments were taken and their in vitro motility was measured in 7 chickens per group. From the remaining birds the content of the stomach, jejunum and ileum were collected for viscosity measurements. Due to enzyme supplementation of the rye-based diet, the viscosity of digesta decreased significantly from 2.6, 17.1 and 35.4 in the stomach, jejunum and ileum digesta in group R, to 1.7, 4.7 and 8.2, respectively, in group RE (P<0.001), the performance of chickens improved, and apparent fat (from 79 to 88%) and protein (from 86 to 87%) digestibilities and the AME_N value of the diet increased (from 13.3 to 14.5 MJ ME/kg DM). The contractile activity of the duodenum and jejunum in response to acetylcholine was higher in group RE than in R, but only in the jejunum was the difference significant. The mean score of stomach distention was 3.05 in group R, 2.32 in group RE, and 2.05 in the control group (P<0.05). High digesta viscosity may negatively affect the development of the stomach and motility of the small intestine in young chickens; lowering viscosity by enzyme supplementation of diets containing viscous NSPs allows reduction of disturbances in digestive tract development.