1. In young rabbits the muscle belly of the tibialis anterior was marked at intervals, either on its surface with indian ink, or in its substance by wires. The intervals between ink marks were measured directly, and those between wires by radiography. After four to seven months the measurements were repeated and the amount and site of longitudinal growth determined. The experiments showed that it occurred fairly evenly throughout the length of the muscle belly.

2. By transfer of the tibialis anterior in front of the crural ligament in young rabbits its course was reduced and the extent of contraction necessary to dorsiflex the foot was increased. The rabbits were killed when fully grown and the lengths of the tendons and muscle bellies of the tibialis anterior of the normal and experimental legs were compared. It was found that in every case the tendon of the experimental muscle was shortened and its belly lengthened in comparison with the normal. It is suggested that the increased length of the muscle belly was determined by the increased distance which it had to contract in order to dorsiflex the foot.

1. Small indian ink marks were made at intervals along the length of tendons in the limbs of young rabbits, and the distance between the marks was measured during the operation. The rabbits were killed two to three months later, and the amount of longitudinal growth that had occurred was determined by re-measuring the distance between the marks.

2. The experiments showed that the whole of the tendon grows interstitially in length, but that maximal growth occurs near the muscle-tendon junction.

3. Histological examination of the tendons and control experiments involving adult tendons indicated that growth was not significantly interfered with by marking the tendons.