The effect of live cells and metabolites of the yeast, Saccharomyces cerevisiae, in the diet on the activity of polysaccharidases in the rumen of heifers was examined. The experiment was carried out on three rumen-fistulated heifers. The control diet was composed of 87.5% meadow hay, 12% concentrate (barley, rapeseed oilmeal, soyaben oilmeal) and 0.5% vitamin-mineral mixture. Two experimental rations consisting of the same components were supplemented with either live S. cerevisiae yeast or their metabolites. The live yeast were supplied at the rate of 10 g · d^–1, metabolites, at 60 g · d^–1. Cellulolytic activity significantly increased when heifers were fed diets with live yeast in comparison with their metabolites before and 4 h after feeding. The yeast preparation significantly decreased xylanolytic activity compared with control animals. Amylolytic activity was significantly increased when live yeast were added compared with animals fed the control diet and yeast metabolites at 0 and 4 h after feeding. The yeast metabolites significantly decreased pectinolytic activity compared with the control group. Chitinolytic activity was higher in control heifers than in animals fed the experimental diets. Our data suggest that administering a yeast preparation may modify the activity of polysaccharidases in the rumen of heifers