Thirty young Black and White Lowland bulls (3 groups of 10 each) were fed a complete feed and meadow hay (0.5 kg/daily) to appetite, individually, starting from an average weight of 150 kg until they reached 430 kg. In the experimental mixtures, the ground barley in the control feed was partly (by 50% in group RJ) or completely (in group R) replaced by ground rape seeds from „00" rape var. Jantar. The daily intake of dry matter from the ration was 7.9, 7.5 and 7.4 kg, and the mean daily liveweight gains were 1156, 1118 and 1080 g in groups K, RJ and R, respectively. The intake of metabolizable energy, crude and digestible protein per kg body gain was higher in group R than in the others. The net energy utilization per kg body gain, calculated according to the INRA system, did not differ significantly among the groups in spite of the increased fat level in the experimental mixtures. However, when it was calculated according to the oat feed units system, it was distinctly lower in group R than in the remaining ones. The digestibility of nutrients, nitrogen retention and VFA concentration and C2/C3 ratio in the rumen fluid were smaller in group R than K, with an increased C3/C4 ratio.