The objectives of this trial were to estimate the body composition of fat and protein and the net energy requirements for maintenance of purebred and crossbred Nellore. Seventy-two young bulls averaging 10 to 11 months of age from four genetic groups were used: 18 Nellore, 18 F1 Nellore x Angus, 18 F1 Nellore x Brown Swiss and 18 F1 Nellore x Simental with initial average weights of 286, 309, 333 and 310 kg, respectively. 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 feed restricted while other three bulls were slaughtered at the beginning of the trial and used as reference. The net energy requirements of maintenance were obtained by regressing heat production (kcal/kg^0.75/day) on metabolizable energy intake (Mcal/kg^0.75/day) extrapolating to zero level of metabolizable energy intake. There was no difference in net energy requirements for maintenance among genetic groups. Body contents of fat, protein and fat concentration (g/kg EBW) increased, respectively, 260.2, 92.6 and 67.8% when body weight increased from 250 to 550 kg whereas body protein concentration reduced 10.9%. The identity test of the non-linear model indicated no significant differences among genetic groups for body composition of fat, protein and energy as well as for net energy requirements for maintenance. The estimated net energy requirement for maintenance was 79.45 kcal/kg ^0.75/day in this study.