The formulation of balanced diets has been a way to optimize the nutrient use by different animals and to get carcasses with high proportion of muscles and proper fat content to attract potential consumer markets. The proportion of these tissues in the carcass can be estimated by measurements in the Longissimus dorsi (L. dorsi) muscle. Characteristics of L. dorsi in feedlot lambs fed different sources of carbohydrates (CHO) and non-protein nitrogen (NPN) were evaluated. It was used 20 non-castrated crossbred lambs with 4 mo of age and 26 kg of body weight. The diets were 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. After cooling, loin of the left half of carcasses was separated and measurements in the L. dorsi were made. The measurements were loin eye width (LEW), loin eye depth (LED), loin eye area (LEA), minimum fat thickness (MinFT), maximum fat thickness (MaxFT). The LEW:LED ratio was also calculated. It was used a completely randomized design with 4 treatments and 5 replicates. Data were analyzed by ANOVA following a 2 × 2 factorial scheme. LED (2.57 ± 0.06 cm), LEA (10.37 ± 0.31 cm²), MinFT (0.76 ± 0.09 mm), MaxFT (1.69 ± 0.12 mm) and LEW:LED ratio (2.12 ± 0.06) were not (P > 0.05) affected by the diets. It was found an interaction (P < 0.05) between CHO and NPN for LEW. The LEW was smaller (P < 0.05) for feedlot lambs fed whey permeate and fast release urea (4.98 cm) than for those fed whey permeate and slow release urea (5.78 cm). The mean LEW was 5.39 ± 0.10 cm. Differences in the LEW did not change the LEA, which may be inferred that the association of whey permeate with slow and fast release urea resulted in the same muscle development. The association of CHO and NPN with concentrates of closest potential degradation rates does not influence the proportions of muscle and fat of lambs fed low quality forage at finishing.