The experiment was carried out on 48 eight-day-old broiler females, kept in individual cages and allocated to 3 groups, 16 birds per group. Two experimental diets were formulated, both contained 800 g/kg of hulless barley cv. Rastik, and were unsupplemented or supplemented with a β-glucanase preparation (Avizyme 1100, 1.5 g/kg). The control diet was based on wheat and soyabean meal and was supplemented with a xylanase preparation. All diets contained 210 g crude protein/kg. The diets were fed for 2 weeks, in the third week 6 chickens from each group were killed and digesta were collected from different parts of the digestive tract for viscosity measurements. The apparent metabolizable energy (AME) of barley was measured in a balance experiment done on 10 birds from each group. Supplementation of the barley diet with β-glucanase increased feed intake by 9% (P < 0.01) and body weight gain by 3.7% and significantly decreased (P < 0.01) the viscosity of digesta, except in the crop and caeca. In the group fed the unsupplemented barley diet, viscosity equalled 1.37, 3.46, 3.93 and 4.78 mPas·s in the digesta from the stomach, jejunum, ileum and caeca, respectively, in the group fed the barley diet with β-glucanase, 0.86, 2.33, 2.91 and 21.55, respectively, while in the control group, 1.01, 1.34, 1.64 and 4.24, respectively. Supplementation with β-glucanase increased the AME value of barley from 10.8 to 13.1 MJ/kg (P < 0.01).