Forty Polish Merino lambs weighing about 17 kg were divided into five groups and fed five isonitrogenous and isoenergetic diets containing 0, 5, 10, 15, and 20% dehydrated lucerne (DL). The animals were fattened to a final live weight of about 36 kg. As the DL concentration in diets increased from Oto 10%, a significant (P≤0.05) parallel rise of mean daily live weight gains from 173 to 191 g/d was observed, followed by a decrease of gains when the DL content increased to 15 and 20%. Lambs fed the 10% DL diet used the smallest amount of protein and energy for a unit of gain. The level of DL in the diet did not have a significant effect on the chemical composition and energetic value of 1 kg empty body weight (EBW). The highest value of the utilization coefficient of metabolizable energy available for growth (k_f) was found in lambs fed diets with 10% DL (0.33), and the lowest in animals from the control group (0.27). Lambs fed 10% DL deposited significantly (P≤0.05) more protein in EBW (29.52 g/d), in wool gain (3.90 g/d) and in total gain (33.42 g/d) than animals from the control group.