During the onset of activation in isolated frog muscle fibres the development of the force-velocity (T-V) relation was determined by imposing single and double ramp releases. The experiments were performed at 3.5-6 degrees C or 19-22 degrees C and at a starting sarcomere length of about 2.25 micron. A velocity- and time-dependent shortening deactivation was shown to exist during the development of contraction. It was found that, early during the tetanus rise, at submaximal levels of activation, the values of T (the steady force exerted by the muscle fibres at any velocity of shortening V lower than V0) were significantly affected by previous conditioning shortening. Conditioning shortening at lower speeds led to potentiation of T and, at higher speeds, to depression. Both these effects were independent of the amount of shortening and, in addition, were not present at the tetanus plateau. At each given time or isometric tension throughout the tetanus rise the values of T. normalized for those determined at the same velocities at the tetanus plateau, were found to be inversely correlated with the actual velocities of shortening. The slope of this relation (a measure of the velocity-dependent shortening deactivation) decreased exponentially with time, attaining, in six fibres at low temperature, 10% of its initial value within 26-73 ms. The results may be explained in terms of a cross-bridge model of contraction by assuming that the rate of development of activation is controlled by the rate of release of the Ca2+ as well as by the velocity at which the muscle fibres are allowed to shorten and in turn by the actual number of attached cross-bridges.