This study tested the effects of free Ca++ on both the small-amplitude mechanical behaviour (dynamic stiffness and phase between 1 and 500 Hz) and the large-scale filament-sliding behaviour (Vmax) of single fibres of chemically-activated glycerol-extracted rabbit psoas muscle. Small-amplitude vibrations (0.1% peak-to-peak of initial length Lo) were used to elicit fw, the frequency for maximum oscillatory work-production per cycle. The unloaded contraction velocity Vmax was measured during the same contractions, using the slack-test method with shortening length steps of up to 10%Lo. These produced nonlinear length-time plots demonstrating that the unloaded contraction velocity was not constant as contraction progressed, but fell with time. This behaviour was approximated by two velocities, V1 the velocity observed for about the first 15 ms and V2 the velocity after this time. V1 and V2 were found to have different sensitivities to Ca++. The value of V2 fell as the level of [Ca++] was reduced, and was linearly proportional to the active tension over the range 0.2 Pmax to Pmax (where Pmax is the isometric tension for saturating amounts of Ca++). In contrast to this V1 remained insensitive to changes in [Ca++] for levels of activation corresponding to active tensions ranging from Pmax to 0.6 Pmax, and then fell as the level of activation was further reduced. It was found that the level of [Ca++] did not affect the magnitude of fw over the range of concentrations yielding active tensions from 0.2 Pmax to Pmax. These results are discussed in terms of the kinetic processes underlying transient readjustments to perturbations from isometric equilibrium.