This study was carried out at the Institute of Animal Nutrition and functional Plant Compounds of the University of Veterinary Medicine Vienna and the University of Natural Resources and Life Sciences in Vienna. The aim of the study was to determine whether the parameters of rumen fermentation and degradability of the nutrients are affected by the use of Saccharomyces cerevisiae. The study was carried out using four trials to determine nutrients degradability and rumen fluid characteristics. The experimental rations included different amounts of yeast (0.0 %, 0.1 %, 0.25 % and 0.50 % yeast). The rations consisted of 50 % first-cut hay, 10 % KUHKORN AUSGLEICH 19 E-100 637 and 40 KUHKORN ENERGIE LEISTUNGSFUTTER E-100 636 from Garant. Focuses of the study were the degradability of crude nutrients on the one hand and some fermentation parameters (VFA, pH, redox potential, gas production) on the other hand. One experiment lasted two weeks, one week to stabilise the fermentation in the vessels and the 2nd week for collecting the samples. The result can be summarized as follows: There was no significant influence of different amounts of yeast on the degradability of dry matter and crude nutrients or energy. The addition of yeast culture to the rations decreased the pH by 0.053. The pH value in all groups was in the optimal range (6.0-6.9). At higher addition rates of yeast, the pH values tended to be lower than 6.1. Ruminal ammonia concentration was significantly different between the groups. The production of total VFA (P=0.047), iso-butyrate (P=0.031), n-butyrate (P=0.05), n-valerat (P=0.002) and caproic acid (P=0,004) was higher in the vessels supplemented with 0.50 % live yeast culture when compared with the vessels supplemented with 0.0 % ,0.1 % and 0.25 % live yeast, respectively. Taking into account that higher amounts of volatile fatty acids provide more energy for the host, the addition of yeast could improve the energy utilization from feed.