It has been previously established that growth and carcass qualities of chicks are modified by genotype and dietary amino acid supply. In this study, we evaluated the effects of lysine deficiency and genetic selection on muscle protein metabolism. Chicks originating from an experimental line selected for breast development (QL) and its control line (CL) were provided ad libitum access to isoenergetic diets containing 20% crude protein but differing in their lysine content (0.75 or 1.01%). Protein fractional synthesis rates (FSR) were measured in vivo in the pectoralis major and sartorius muscles of 3-wk-old chickens (flooding dose of [3H]phenylalanine). Fractional breakdown rates (FBR) were estimated as the difference between synthesis and deposition. Lysine deficiency reduced (P < 0.001) growth performance and muscle weights and increased (P < 0.05) muscle FSR, capacity for protein synthesis (muscle RNA:Protein, Cs) and FBR. Although QL birds grew faster and had heavier pectoralis major muscles than CL birds (P < 0.05), there was no line difference in sartorius weight (P = 0.15). No difference between the lines was observed in sartorius protein metabolism (P > 0.14). In the pectoralis major muscle, chicks of both lines receiving an adequate lysine intake also exhibited similar protein turnover rates. However, in chicks fed the lysine-deficient (0.75% lysine) diet, FSR and Cs were higher in QL than in CL chicks (P < 0.05), and FBR tended (P = 0.07) to be higher in QL chicks. This increased protein turnover in the QL birds on the lysine-deficient diet suggests that the responsiveness of muscle protein metabolism to amino acid supply is modified by genetic selection for breast development.