Evaluated was the influence of live cells and metabolites of yeast (Saccharomyces cerevisiae) in the diet on number of ciliates, concentration of volatile fatty acids (VFA) in the rumen and biochemical parameters of blood of heifers. The experiment was carried out on three rumen-fistulated heifers. The animals were fed diet consisting of 88% meadow hay and 12% concentrate. A dose of 10 g of live yeast or their metabolites - 60 g, were introduced into the rumen. The preliminary feeding period of animals lasted three weeks and was followed by sampling of rumen fluid (RF), rumen contents (RC) and blood. The number of protozoa in RC and concentration of VFA in RF as well as total protein, triacylglycerol, total cholesterol, HDL (high density lipoprotein), LDL (low density lipoprotein) and VLDL (very low density lipoprotein) in blood serum were determined. Supplementation with metabolites of yeast significantly increased the number of genus Entodinium compared to animals fed control diet or live cell of yeast. The number of representatives of the genus Diplodinium was similar in heifers fed control diet or metabolites of yeasts and was significantly higher than when the live yeast was applied to the diet. The number of ciliates from genus Ophryoscolex and Dasitricha significantly increased, when heifers were fed diet supplemented with live cells of yeast. Addition of fungal preparations to the diets increased RF pH compared to animals fed control diet. The administration of yeast metabolites to heifers increased molar concentration of acetate and acetate to propionate ratio, but decreased molar proportions of propionate and butyrate. The total protein, triacylglycerol and total cholesterol concentrations decreased significantly when live yeast cells were added comparing to animals fed control diet or metabolites of yeast.