Effect of castration on the live weight and the growth rates of skeletal muscle, abdominal fat, intermuscular fat, skin, bone and viscera were investigated in this study. Capons and cocks of New Hampshire (NH), Barred Plymouth Rock (BPR) and capons of BPR treated with testosterone propionate (TP) from 26 to 31 weeks of age were used. Live weights and the various weights of each organ and tissue were compared between the experimental groups at 20, 30 and 31 weeks of age. The results obtained were as follows: 1) Live weights of capons were the same as those of cocks in NH and BPR breeds during experimental periods. The weights of TP treated capons were increased gradually after the administration of TP and became 250 g heavier than those of BPR cocks and the capons at 31 weeks of age. 2) Castration had no effects on the growth of skeletal muscle until 20 weeks of age, but showed a marked effect on it after 20 weeks of age. Skeletal muscle weights of capons were apparently smaller than those of cocks at 30-31 weeks of age in NH and BPR breeds. TP administration recovered completely the decrease of these weights after castration, and became equal to those of cocks at 31 weeks of age. 3) Castration had also a marked effect on the growth of abdominal and intermuscular fat after 20 weeks of age in NH and BPR breeds. TP administration decreased the weights of abdominal and intermuscular fat in BPR capons, but these weights were heavier than those of the cocks at 31 weeks of age. 4) Castration exerted some effects on the growth of skin. These weights were increased following the castration in NH and BPR breeds, and became obviously to be heavier than those of cocks at 30-31 weeks of age (21-22 weeks after castration). 5) No effects were found on the growth rates of viscera and bone following the castration and the TP treatment. In conclusion, castration has a remarkable effect on the muscle production and the fat deposition. It decreases the skeletal muscle weight and increases the fat deposition. It is supposed that TP administration on the capon stimulates the growth of skeletal muscle.