The triceps surae muscle-tendon units of rabbits were tested in vivo by a passive extension technique using an MTS Bionix 858 machine. The effect of nerve function on the failure mechanism was evaluated. The sites of failure were always at the muscular portion near the musculotendinous (MT) junction. It always occurred within 0.1 to 1.0 mm of the distal MT junction of the soleus muscle and within 5 to 10 mm of the distal MT junction in the lateral head of the gastrocnemius muscle. In the medial head of the gastrocnemius muscle, it always occurred within 5 to 10 mm of the proximal MT junction. The loss of nerve function significantly reduced the peak force and the absorption of energy before the point of peak force, but it produced no significant changes in the failure length and the total energy absorption. The functioning nerve might have a protective mechanism which works on the muscle-tendon unit by increasing the peak force and the energy requirement before the muscle is injured.