1. Passive length-tension curves were established for cat soleus muscles that had been immobilized in different positions. Muscles that had been immobilized in the lengthened position showed no difference in their length-tension properties to those of normal muscles. However, those immobilized in the shortened position showed a considerable decrease in extensibility.2. Muscle fibre length, sarcomere length and the total number of sarcomeres along single teased fibres were also determined for muscles immobilized in different positions. Soleus muscles immobilized in the lengthened position were found to have 20% more sarcomeres in series than normal muscles whilst those immobilized in the shortened position had 40% less than normal muscles.3. When the plaster casts were removed from muscles that had been immobilized in the shortened position, the length-tension curves and sarcomere number returned to normal within 4 weeks. Muscles that were immobilized in a shortened position and then immobilized in a second position were found to rapidly adjust to the second position with respect to their passive length-tension properties and sarcomere number.4. A change in the number of sarcomere in series seems to be the way in which the sarcomere length of the muscle is adjusted to its new functional length. The change in the length-tension properties which accompanies a decrease in sarcomere number appears to be the mechanism which prevents the muscle from being overstretched.