Glycogen phosphorylase is a major sarcoplasmic protein in chicken pectoralis muscle, constituting approx. 4% of the total protein complement. In slow-growing layer chicks phosphorylase accumulated in parallel with muscle accretion, but in fast-growing broiler chicks the concentration of phosphorylase in the muscle increased (from 5 to 8 mg/g wet wt.) with time. In a 5-week period, the total amount of phosphorylase in the pectoralis muscles increased 18-fold in broiler chicks (from approx. 75 to 1400 mg total), but only 3-fold (from approx. 100 to 270 mg total) in layers. Pyridoxal phosphate, the cofactor of the enzyme glycogen phosphorylase, was used as a specific label to measure the rate of degradation of the enzyme in the pectoralis muscle of growing broiler and layer chickens in vivo. In young animals, the fractional rate of phosphorylase synthesis was similar in broiler and layer chickens (approx. 15%/day), but the rate of degradation in layers (5%/day) was 5-fold higher than in broilers (1%/day). As the animals aged, the rate of synthesis decreased, but more so in layers than in broilers. The rate of degradation of phosphorylase also decreased in layers, but in broilers it remained at the low level seen in young animals. The dramatically higher rate of phosphorylase accretion in the pectoralis muscles of the broilers is therefore achieved by an initial lower rate of degradation combined with a sustained difference between rates of synthesis and degradation.