Corresponding muscles from the right and left legs of the frog were isolated in Ringer''s soln. or in moist chambers. The length of survival of the muscles was determined by applying to them at intervals single induction shocks. Corresponding muscles which were both kept loose died at approximately the same time. If one muscle of a pair was kept under stretch while the other was kept loose, the stretched muscle survived longer than its loose control in 75 out of 77 expts., the difference in favor of the stretched muscle being 6.6 hrs. on the average of 55 cases. The result is explained on the assumption that stretch mechanically supports the physical structure (polarity) of the contractile units requisite for contractility, delaying, thus, the irreversible breakdown of that structure.