Tension decay following stretch (stress relaxation) has been studied in resting long-fibred smooth and striated muscles. In both types of muscle, tension decays rapidly at first and then more slowly; the decay curve can be resolved into two exponential components, neither of which is significantly affected by changes in temperature or speed of stretch. It is shown that the half-times of fast and slow components of tension decay are related to the intrinsic speed of muscle. A model is proposed which imitates the properties of preshortened resting muscle at lengths where it shows negligible resting tension.