Two experiments were carried out to study the effect of supplementing soyabean meal with methionine, at four different protein levels, on plasma free amino acids of growing chickens. Two blood sampling methods were used: single bleeding, where each bird is bled once (Experiment 1) and serial bleeding where each bird is bled consecutively every four days (Experiment 2). Following a randomized paired-feeding design based on metabolic body weight (W^0.75), 11-days old White Rock male broilers were allocated, for an experimental period of 20 days, to one of two isoenergetic, semisynthetic diets containing 60, 120, 180 and 240 g protein/kg diet. All diets were based on soyabean meal either unsupplemented or supplemented with 2 g/kg DL-methionine. Thirty-two and eight chickens were used in Experiments 1 and 2, respectively. Interactions between sampling method and either supplementation or protein level were not significant for most essential amino acids. Methionine supplementation of the soyabean meal diets elicited an increase of methionine and a significant decrease of threonine, valine, leucine and lysine in plasma, irrespective of sampling method or protein level (except no effect of methionine supplementation at 24% CP for plasma free lysine), while basic and aromatic amino acids remained unaltered. Increasing dietary protein level four fold augmented plasma levels of arginine, threonine, valine and methionine, irrespective of sampling method or supplementation effects. Threonine was the most sensitive amino acid to the effect of dietary protein level, increasing its concentration 3 fold. In conclusion, multiple and single bleeding methodologies were both sensitive to evaluate the improvement of dietary protein quality after methionine supplementation.