The levels of blood metabolites of feedlot lambs fed low quality forage and different sources of carbohydrate (CHO) and non-protein nitrogen (NPN) in the concentrate feed was evaluated. Twelve non-castrated male lambs with 4 mo of age and 23.5 kg BW were used. The experimental design was 4 × 3 incomplete Latin square replicated 3 times, with 4 treatments (diets) and 3 periods of 21 d. 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 fiber (72.4% NDF) and low protein (4.9% CP). Four concentrate feeds 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), and fed at 1.5 times the ME requirement. At the end of each period, blood samples were collected after fasting (feed for 14 h) to assess the serum levels of total proteins (TP), albumin (Alb), creatinine (Creat), urea, glucose (Glu), cholesterol (Chol), triglycerides (TG) and β-hydroxybutyrate (BHBA). Data were analyzed by ANOVA and means were compared by LSD test (P < 0.05). Serum levels of TP (61.7 g/L), Alb (31 g/L), Creat (0.91 mg/dL), Glu (64 mg/dL) and BHBA (0.12 mg/dL) were not affected by the diets. Serum urea was higher in lambs fed corn and fast release urea (29.9 mg/dL), and lower in those fed whey permeate independent from NPN source (25.4 mg/dL). High levels of serum urea can be related to high production of ammonia in the rumen due to the differences in degradation rates between corn and fast release urea, and by the higher CP of corn (9%) compared with whey permeate (3%). Maybe the whey permeate was fermented faster than corn resulting in greater MP production and less ammonia in the rumen. Serum levels of Chol and TG were higher in lambs fed corn (45.2 and 24.6 mg/dL) compared with those fed whey (34.6 and 17.1 mg/dL), which was caused by the lipid fraction of corn (5.4%). The inclusion of corn or whey permeate in the concentrate feed affects the protein and energy metabolism of lambs fed low quality forage.