Based on the production and interrelationship of volatile fatty acids (VFA), the mathematical method according to Orskov et al. (1968) was used to determine the efficiency of VFA production, and/or conversion of the energy of hexoses contained in fodders into VFA energy in rumen. VFA were separated by gas chromatography. The energy yield of VFA production in the rumen contents of wethers was averaged from the samples taken one, three and five hours after feeding. Whethers were fed 11 experimental diets, in which a part of bulk fodder (5-20%) was replaced by treated or untreated sawdust, and/or a diet without sawdust. The energy yield varied from 73.12 to 76.51% and the maximum values were achieved with the diets containing no sawdust. Compared with the diets with sawdust addition, the differences are statistically significant (P less than 0.05). The addition of the treated beech sawdust to the same diet, in comparison with untreated sawdust resulted in a higher energy yield of VFA production, however, with no statistical significance. The ratio of acetate to propionate was in a direct but negative relationship to energy yield of VFA production (n = 44, r = -0.905, P less than 0.001). Therefore the diets rich in cellulose, which cause an increase in the molar percentage of acetic acid and a subsequent increase in the ratio of acetate to propionate, might be responsible for the energy losses in the form of methane and can result in the decrease in total energy balance (Orskov et al., 1968).