Abstract: This study aimed to evaluate the growth performance, blood metabolites, insulin and testosterone and wool traits of growing lambs fed with a diet containing low and high level of a synbiotic. A total of 14 growing Najdi male lambs (aged six months and weighed 22.02±0.41 kg) were randomly divided into three groups; control group (n = 4) fed a basal diet (C), low level group (n = 5) fed basal diet plus 20 ml of synbiotic (L) and the high level group (n=5) fed basal diet plus 40 ml of synbiotic (H). Synbiotic was given orally for a period of 12 consecutive weeks. Animals were bled for serum once a week throughout the experiment. Serum total protein, albumin, glucose, cholesterol, insulin and testosterone were quantified. Wool traits were also studied. Growth rate was determined of the biweekly routine body weights. Lambs given high level of synbiotic showed the highest growth (p<0.01) gain (13.74 kg) and had the heaviest weight (35.76 kg) followed by lambs given low level (8.75 and 30.87 kg, respectively) and then by the control lambs (7.33 and 29.36 kg, respectively). Total protein concentration was significantly higher in H-lambs compared with C-lambs (7.93 vs. 6.43 g dL-1, respectively) with intermediate level in L-lambs (6.72 g dL-1). Blood cholesterol decreased (p<0.05) in L-lambs (33.67 mg dL-1) compared with C-lambs (49.32 mg dL-1) whereas H-lambs gave a value of 44.18 mg dL-1 in blood of lambs with high level of synbiotic. Glucose in blood was not different (p<0.05) due to either L (58.47 mg dL-1) or H (66.14 mg dL-1) synbiotic compared with C (61.43 mg dL-1) lambs, even though there found a significant (p<0.05) difference between L and H on glucose concentration. Insulin levels significantly (p<0.01) decreased at conclusion of the experiment as compared with initial levels. The highest (p<0.01) level of insulin was found in H-lambs (27.3 μIU mL-1), however low level of synbiotic was not differ than control. Testosterone increased (p<0.05) by about 52% in H-lambs (5.0 ng mL-1) than in control (3.3 ng mL-1) lambs. The high level of synbiotic exhibited the shortest staple, lowest crimp and intermediate diameter of wool fibre.