SUMMARY 1. The effect of 15 days' treatment with purified pituitary growth hormone (GH) on the composition of the protein of the quadriceps muscle was studied in adult female rats. 2. The muscle protein was divided into three fractions, the sarcoplasmic protein, the myofibrillar protein, and collagen and elastin. No differences were found in the proportions of the three fractions in normal and in GH-treated rats. 3. There are two components of muscle with adenosine triphosphatase (ATP-ase) activity: the myofibrils and the sarcoplasmic granules. These two activities and the total ATP-ase activity were assayed in homogenates from GH-treated and control rats. The only alteration observed in the rats receiving GH treatment was a diminution in the amount of myofibrillar ATP-ase per gram of fresh tissue.