Abstract: Thirty lambs (15 rams and 15 ewes), 4-5 m old, pre-experimental mean body weight of 20.9�0.5 kg were divided into five treatment groups of six lambs each, (3 ewes and 3 rams) in 2 x 5 factorial randomized complete design experiment. These were used to investigate the effects of concentrate mixtures of varying energy and protein levels on voluntary feed intake, growth rate and nutrient utilization for 84 experimental days. The five concentrate mixtures were designated as HPHE; HPME; HPLE; MPME and LPME. First letter in each concentrate mixture designates protein level while the third letter designates energy level (H, high, M, medium and L, low). The letters P and E stands for protein and energy, respectively. CP and GE content of HPHE, HPME, HPLE, MPME and LPME concentrate mixtures were 16.5, 16.5, 16.5, 15.0 and 13.0%; 21.6, 19.0; 17.6 and 19.0 MJ/kg, respectively. Significant differences (P<0.001) were observed among lambs in DMI of concentrate mixtures. Lambs on HPLE had higher concentrate intake (467g) while lambs on HPHE were lowest at 357 g/head/day. Total DMI (concentrate + forage) was significantly different (P<0.001) for lambs in the different treatments. Sex has no significant effect (P<0.05) on DMI. ADG of ram lambs were 83, 140, 117, 131 and 81 compared to 81, 101, 101, 91 and 79 g/d for ewe lambs, respectively on HPHE; HPME, HPLE, MPME and LPME concentrate mixtures and these were significantly different (P<0.05). ADG was numerically higher in ram lambs on MPME and HPHE and lowest on LPME. Better-feed efficiency was obtained in lambs on HPME, MPME and HPLE treatments. Daily protein intake was significantly different (P<0.001) among lambs in the different treatments. Blood urea-N and blood glucose concentration in lambs on HPHE, HPME, HPLE, MPME and LPME concentrate mixtures significantly increased (P<0.001) at the post experimental period. Treatments and sex had significant effects on nutrients digestibility and rams were higher than ewes in the digestibility of DM, OM, CP, ADF, ADL and hemicellulose (P<0.001). Sex and dietary treatments have influence on DMI, nutrient digestibility and growth rate of lambs. Based on all parameters tested, available data therefore demonstrate that dietary CP that ranged between 15 to 16.5 % CP (daily protein intake of 6.8 – 7.7 g/kg ^0.75/day); and 17.6 to 19.0 MJ GE/kg BW (10.9 – 11.8 digestible energy KJ/kg DM), respectively will satisfy the requirements of ewe and ram lambs of the Fiji Fantastic sheep for maximum growth in the tropical environment of Fiji. In conclusion all the treatments with the exception of the LPME concentrate mixture are likely to ensure adequate supply of available nutrients and energy for rumen degradation. However, the HPME treatment that had 16.5 % CP (daily protein intake of 6.8 g/kg ^0.75/day) plus 19.0 MJ GE/kg BW (11.7 digestible energy KJ/kg DM) was the best because the lambs on it had better average live weight gain at 121 g/lamb/d.