1. The time course of stress relaxation following stretch was investigated in the taenia coli of the guinea pig in the relaxed (Ca2+ free bath solution with D 600) and in the contracted state (depolarization by KCl or K2SO4 in excess). The mechanical tension was standardized with respect to the volume of the samples. 2. The tension obtained by a constant stretch of 2.5 mm of the 5-15 mm samples was highest after K2SO4-depolarization (K+ = 180 mval/l) and lowest in the relaxed muscles. Muscles contracted by KCl in escess (K+ = 60 mval/l) showed intermediate values. 3. The decrease of tension by stress relaxation to a nearly constant residual value is rapidly observed in contracted muscles during about 10 s. For the relaxed muscles the same changes are slower and take place in nearly 1/2 h. 4. The relations between the extent of stress relaxation (R) and its derivative in time (dR/dt) can be expressed by a hyperbolic function. Analogous behaviour was already noted in previous studies primarily on creep. 5. The different behaviour of the muscle in the different bath solutions used is discussed with respect to the mathematical relations just mentioned. 6. Long term active reactions of depolarized muscles following stretch and stress relaxation are noted.