1. Segments of skinned frog muscle fibres were immersed in solutions in which the calcium ion concentration, and therefore the steady isometric force, was controlled with an EGTA buffer system. The contraction kinetics were measured when the load was quickly reduced to values less than the isometric force.2. The velocity of shortening following release from steady force was a hyperbolic function of relative load. The minimum quick displacement required to reduce the force from the steady isometric value to half this value was about 1% of the fibre segment length.3. The relative force-velocity relation was independent of pCa in the range 5.0-6.75. Thus calcium ions appear to control the number of sites at which cross-bridges can be formed but have no significant effect on the kinetic properties of an individual bridge.4. Cross-bridges along the length of a myofilament appear to act independently of each other.5. The force-velocity relations reported for intact muscle fibres during relaxation, when compared with those of skinned fibres at different calcium levels, indicate that the time course of the fall in force after a stimulus is essentially that of calcium removal from the myofilaments.