The energy metabolism was measured by means of indirect calorimetry in growing bulls with mean live weights of 290-420 kg and in wethers with mean live weights of 40-60 kg. The animals were fed 12 (bulls) and 8 rations (wethers) with high variation in the nutrients composition and with a range of energy metabolizability (q) between 0.74 and 0.47 on 3 steps of nutrition level (deposition, maintenance, underfeeding) as a rule. The energy digestibility of the 12 rations tested in growing bulls on the nutrition level maintenance was measured between 81.9 and 56.4% and that of the 8 rations tested in wethers was measured between 82.6 and 55.6%. On the high and middle nutrition levels the data of energy and nutrients digestibility in growing bulls and wethers were in good agreement, except the significant lower digestibility of crude protein in cattle. On the low nutrition level digestibility of energy, crude protein, crude cellulose and N free residual substances in wethers was 3-5 digestibility units lower than in bulls. The increase of nutrition level by 1 unit lowered the energy digestibility in bulls by 4.2 and the nutrients digestibility about 4-8%-units (0.7 for starch). In sheep the effect was not so high as in cattle. There were differences in the rumen physiological parameters between the animal species. In the growing bulls' rumen liquid the part of acetate was higher and the parts of propionate and butyrate were lower. The content of NH, was higher in sheep. The increase of nutrition level caused higher amounts of volatile fatty acids in the two animal species with lowering the part of acetic acid and raising the parts of propionic and butyric acid.
