The objectives of this trial were to estimate gain composition and energy and protein requirements for weight gain of purebred and crossbred Nellore. Sixty 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 was adopted and bulls from the four genetic groups were fed diets containing: 30, 40, 60 and 70% of concentrate. Three animals from each genetic group were slaughtered at the beginning of the trial and used as reference for initial body composition. The net protein and energy requirements for gain of 1 kg of empty body weight (EBW) were estimated by the equation Y′ = a. b. X ^(b-1), where a and b are the parameters of the regression equations and X is the EBW of the animals. The identity test of the models indicated no significant differences among genetic groups for the net energy and protein requirements for weight gain. It was observed a decrease of 10.6% and an increase of 37.8%, respectively, in the protein and energy requirements for body weight gain between 250 and 550 kg. These changes are result of the increase in the content of fat and reduction in that of protein as EBW increased. The net requirements of protein and energy for body weight gain were estimated as 143.5 g and 4.7 Mcal, respectively, for a body weight of 450 kg.