1. The force-velocity relationship and the stress-strain curve of the so-called series elastic component (s.e.c.) of frog sartorius, semitendinosus and gastrocnemius have been determined during shortening against a given force (isotonic quick-release) and at high speed (controlled release): (a) from a state of isometric contraction and (b) after stretching of the contracted muscle. In both cases the muscle was released from the same length: this was usually slightly greater than the muscle's resting length.2. The muscle released immediately after being stretched is able to shorten against a constant force, P, equal to or even greater than the isometric force, P(0), at the same length. When the force P applied to the muscle is reduced below P(0) the velocity of shortening is greater after stretching, and the force-velocity curve is therefore shifted along the velocity axis: the shift is maximal when P is near to P(0) and it decreases rapidly with decreasing P.3. The extent of shortening of the s.e.c. required to make the force fall from P(0) to zero is 50-100% greater when the muscle is released immediately after stretching than when it is released from a state of isometric contraction. This difference is found by using either the controlled release method or the isotonic quick-release method.4. If a time interval is left between the end of stretching and the onset of shortening of the contracted muscle (controlled release method), the length change of the s.e.c., for a given fall of the force, is reduced and approaches that taking place when the muscle is released from a state of isometric contraction.5. Curare does not affect the results described above, indicating that these do not depend on modification of the neuromuscular transmission.6. It is concluded that stretching a contracted muscle modifies temporarily: (a) its elastic characteristics, as shown by the greater amount of mechanical energy released for a given fall of the force at the muscle's extremities, and (b) its contractile machinery, as it is suggested by the change of the force-velocity relationship.