The influence of live Saccharomyces cerevisiae cells and their metabolites on the number of ciliates and some fibrolytic and amylolytic enzymes, together with the effect on the concentration of volatile fatty acids (VFA), and disappearance of dry matter (DM) and structural carbohydrates in the rumen of three goats was examined. The control diet (1.2 kg DM·d^-1) was composed of hay (63%), barley meal (31%), and soyabean meal (4%). Two experimental rations consisting of the same components supplemented with either live Saccharomyces cerevisiae (CNCM I-1077) cells or their metabolites (Diamond V XP). The additives were supplied at the rate of 3 and 25 g·d^-1, respectively. The experiment was carried out in a 3 x 3 Latin square design. Enrichment of the control diet with yeast metabolites increased (P<0.05) the total number of protozoa and the number of Diplodinium from 115 to 146×10⁴ and from 2.5 to 6×10⁴·g^-1digesta, respectively. Conversely, the number of representatives of the genus Isotricha decreased over eightfold regardless of the additive used. The activity of carboxymethylocellulase was about 9.4 μM released reducing sugars·g^-1DM·min^-1 irrespective of diet, whereas xylan digestion increased from 57.5 to 69.7 and 70.4 μM released reducing sugars·g^-1DM·min^-1 when live yeast (P<0.05) or their metabolites (P<0.01) were added, respectively. On the other hand, amylolytic activity decreased from 58 to 50 μM released reducing (P<0.05) sugars·g^-1 DM·min^-1 when the control diet was supplemented with viable Saccharomyces cerevisiae. Live yeast cells increased the disappearance of neutral detergent fibre (NDF) from 38 to 44.7% after in sacco incubation of hay in the rumen for 16 h (P<0.05). The disappearance of neither DM nor acid detergent fibre was affected by the diet. Yeast metabolites decreased (P<0.01) total volatile fatty acids (VFA), whereas live yeast had no effect. Enrichment of the control diet with live yeast increased (P<0.05) the molar proportion of acetate from 62 to 64% of total VFA. Both additives elevated the molar proportion of butyrate from 10.5 to 11.9 and 11.3% of total VFA, respectively (P<0.05) and lowered that of propionate. The acetate/propionate ratio in the rumen of goats fed the control diet and the diet supplemented with either live yeast (P<0.01) or their metabolites (P<0.05) was 2.7, 3.1 and 2.9, respectively. The yeast metabolites increased ruminal pH from 6.5 to 6.7. No changes in acidity were found when live yeast were added.