Aim of this work was to evaluate the effect of temperament on certain metabolic parameters (cortisol, glucose, serum fructosamine, triglyceride, albumin, total protein, urea), as well as daily weight gain in fattening lambs. Eight calm (score 1) and eight nervous (score 4 and 5) German Mutton Merino ram lambs were involved in the investigation on a sheep farm. Lambs’ temperament was evaluated by a temperament test (movements of animals were assessed in 5-score system during weighing – from 1: calm, to 5: nervous - while spending 30 sec on the scale) at three times: 1st, 20th and 40th days of the 40-day experiment. The blood samples were taken immediately after temperament scoring. The calmer lambs had lower (P<0.05) concentrations of plasma cortisol (17.11 nmol/l), serum fructosamine (346.53 μmol/l), triglyceride (0.214 mmol/l), and urea (3.31 mmol/l) as well. In addition calm lambs had higher (P<0.05)daily weight gain (466.67 g/day), compared to the nervous animals (25.13 nmol/l, 503.76μmol/l, 0.275 mmol/l, 5.51mmol/l and 345.36 g/day, respectively). These results suggested that lambs’ blood biochemical parameters influenced by temperament, nervous and calm lambs differ in carbohydrate, lipid and protein metabolism, demonstrated by an increase in urea, triglyceride and serum fructosamine in nervous lambs.