Abstract. The efficiency of rumen fermentation was studied in lambs and wethers using 14 experimental rations with and without monensin sodium. The 7 pairs of rations (the first a control diet, the second a monensin treated one) consisted of hay and concentrate at ratios of 60:40, 40:60, 100:0, 50:50 (lambs), 100:0, 75:25 and 50:50 (wethers), respectively. The molar % of acetic, propionic, butyric and valeric acids were taken into stoichiometric calculations of rumen fermentation. In most of the rations monensin increased rumen fermentation efficiency. Of all lamb and wether rations the highest efficiency was recorded in the monensin treated ones consisting of 40 % hay and 60 % concentrates and 50 % hay and 50 % concentrates, respectively. The high fermentation efficiency was probably due to the lowest methane production observed with these rations. This means that monensin increased the amount of metabolizable energy by decreasing methane energy losses. The increase in fermentation efficiency is associated with a high production of propionic acid and in wethers also with an increase in the gross efficiency of feed conversion.