Despite of the huge availability of agricultural lands and good weather conditions to forage growth in Brazil, feedlots have been an effective way to increase animal production. The aim of this trial was to evaluate the performance and feeding efficiency of feedlot lambs fed diets with different non-nitrogen protein (NPN) and carbohydrates (CHO) sources in the concentrate. We used 20 non-castrated lambs with 4 mo of age and 26 kg of body weight. Lambs were fed ad libitum for 45 d with diets composed by 57% of grass hay (Brachiaria hybrid) and 43% of concentrate. The hay was harvested at late mature stage with high (72.4%) neutral detergent fiber and low (4.9%) crude protein. Four concentrates were formulated from the combination of 2 CHO (ground corn: starch; whey permeate: lactose) and 2 NPN (fast and slow release urea) sources. The diets were isoenergetic (2.32 Mcal/kg ME) and isoproteic (11% CP). Lambs were slaughtered at the end of trial after 45 d with a mean body weight of 30.4 ± 1.0 kg. A completely randomized design was used with 4 diets and 5 replicates. Data were analyzed by ANOVA following a 2 × 2 factorial scheme. Body weight gain (BWG), average daily gain (ADG), dry matter intake (DMI), gross feed efficiency (GFE) were analyzed. There was no interaction between CHO and NPN source (P > 0.05) and NPN had no effect on any variables (P > 0.05). Animals that were fed corn had higher BWG (5.28 vs. 3.54 kg), DMI (1.010 vs. 0.937 kg day⁻¹), ADG (117.30 vs. 78.60 g day⁻¹) and GFE (116.80 vs. 84.3 g kg⁻¹ DM) than those fed whey permeate. The best performance and high intake of diets with corn can be related to a possible microbial growth limitation and the depression of fiber degradation in diets containing whey permeate. Lactose provides less carbon than starch on a mass basis and has a higher potential for energy-spilling due to its faster fermentation. The source of NPN in the concentrate feed had no effect on performance of finishing lambs fed a low quality diet. Ground corn was a better ingredient for concentrate feed in diets containing low quality forage.