The objective of this trial was to estimate the efficiency of utilization of metabolizable energy (MEEU) for maintenance (K_m) and weight gain (kg) of feedlot purebred and crossbred Nellore. Seventy-two young bulls averaging 10 to 11 months of age from four genetic groups: Nellore, F1 Nellore x Angus, F1 Nellore x Brown Swiss and F1 Nellore x Simental and initial average body weights of 286, 309, 333 and 310 kg, respectively, were used in this study. A completely randomized design with a 4x4 factorial arrangement was adopted and bulls from the four genetic groups were fed diets containing: 30, 40, 60 and 70% of concentrate on dry matter basis. Three animals from each genetic group were assigned to the maintenance group and other three were slaughtered at the beginning of the trial. The intake of metabolizable energy for maintenance (MMEI), expressed in Kcal/kg^0.75, corresponded to the point in which the coefficient between fasting heat production (FHP) and MMEI was closer to 1. The MEEU for maintenance (K_m) was estimated by dividing FHP by MEI_m while that for gain (K_g) was estimated by regressing retained energy (kcal/kg^0.75) on MEI_g. The ME requirements were obtained by dividing the net energy requirements by K_m. Genetic group and dietary level of concentrate did not significantly affect K _m and K_g that averaged 0.67 and 0.40, respectively. The ME requirements for gain (ME_g) and the total ME requirements (ME _t) followed the increase in body weight. However, the requirement of ME_t and ME_g per unit of empty body weight (EBW) decreased as body weight increased indicating a greater efficiency of metabolizable energy utilization.