To explore the mechanisms involved in the genetic control of muscle growth and protein gain, protein metabolism was assessed in the pectoralis major muscle of two chicken lines selected for either fast or slow growth. Protein synthesis was measured in vivo at various ages from 1 to 4 wk, using a flooding dose of L-[4-3H] phenylalanine. Protein degradation was estimated as the difference between synthesis and deposition. Over the experimental period, BW were about 2-fold greater (P < 0.001), and pectoralis major muscle weights were 2.4- to 3.6-fold higher (P < 0.001), in chicks from the fast-growing line (FGL) than those from the slow-growing line (SGL). Independent of age, absolute rates of protein deposition, synthesis, and breakdown were higher in FGL than in SGL chickens. Fractional rates of muscle protein synthesis clearly decreased with age. When comparing birds of the same age, fractional rates of muscle protein synthesis tended to be lower in the FGL. Fractional degradation rates (KD) were significantly lower in FGL chickens during the first 2 wk of post-natal growth, whereas KD were similar between lines in older chickens. In this experimental model of chicken lines divergently selected for BW, the greatest line-related difference in muscle protein metabolism was in KD, and was observed in the early growth phases.