The longitudinal growth of M. tibialis anterior was measured after tenotomy in young rabbits. After simple transection, while the tendon remained interrupted, the experimental muscle belly was shorter than the control; following tendon regeneration in about one month, muscle growth accelerated, the belly becoming almost or completely as long as normal by the adult stage. Gross displacement of the transected tendon combined with resection of the tendons of M. extensor digitorium longus prevented regeneration, and longitudinal muscle growth virtually ceased. Intermediate tendon displacement resulted in imperfect repair and diminution of muscle growth. Usually the length of the experimental muscle belly with the foot plantar-flexed was approximately proportional to its length when exerting its maximum developed tension and to the range of lengths through which it exerted tensions comparable with those of the control. However, the latter was disproportionately reduced when growth ceased after tendon displacement, probably because the muscle fibre fasciculi occupied a decreased proportion of the belly length. The relative length of fasciculi isolated by maceration and teasing of muscles following simple tendon transection was normal, as was their sarcomere length in plantar flexion. The maximum developed tension the muscle exerted tended to be slightly diminished when tendon repair was good, with greater diminution after poor repair. It was increased when the tendons of extensor digitorum longus were resected, probably because growth in the strength of tibialis anterior was stimulated by its isometric contractions against adhesions in attempting to dorsiflex the foot.