The tension transients following step length changes imposed on tetanized muscle fibres during the steady phase of force response to lengthening were determined at different velocities. At low velocities the early partial recovery after a step was smaller and slower than under isometric conditions, while the speed of the final total recovery was faster. The degree of depression of the early recovery and the speed of the final recovery increased with the lengthening velocity. At a given lengthening velocity the speed of the total recovery depended on size and direction of the steps, increasing from the region of the larger releases to that of the larger stretches. The changes in the early partial recovery are explained qualitatively by the theory of Huxley and Simmons (1971), while the changes in the speed of the final recovery are explained by assuming that detachment of cross-bridges is negligible until a certain range of cross-bridge strain is reached, and then increases rapidly. It is also necessary to assume that cross-bridges detached in this way re-attach much more rapidly than when they detach on completion of their cycle during shortening.