1. After Nr-methylhistidine (Nr-MH) distribution among the various organs or the tissues was determined in male broiler chickens of 15 d of age, the rates of degradation of myofibrillar proteins in male layer and broiler chickens at different stages of growth were determined by means of Nr-MH.
2. About 75 and 8% of the total Nr-MH in the tissues occurred respectively in skeletal muscle and stomach, and most of the remainder in the intestine and the skin.
3. The rates of degradation of myofibrillar proteins in the male layer and broiler chickens of 21, 42 and 63 d of age were calculated to be 6.1, 4.5 and 2.4%/d (layer) and 5.0, 2.8 and 0.9%/d (broiler) respectively. These calculations involve the assumption that 80% of the total excreted Nr-MH was derived from skeletal muscle.
4. The results strongly indicate that the rapid growth of the broiler chicken is facilitated by the reduced rate of protein degradation.