The mechanical response of fully activated muscle bundles (one to five fibers) to sinusoidal length perturbation ( approximately 0.4% L(0)) was studied as a function of MgATP concentration. The frequency response (0.25-167 Hz; corresponding to 1 ms time resolution) of chemically skinned rabbit muscle fibers was resolved into three exponential rate processes, (A), (B), and (C). At 20 degrees C, the apparent rate constants associated with the fast exponential lead (2pic = 388-588 s(-1)) and the oscillatory work (2pib = 59-116 s(-1)) both increase with increment of the MgATP concentration from 1 to 5 mM, and they both saturate for further increase. Over the whole range of MgATP concentrations the slow exponential lead (2pia = 9-7 s(-1)) remains constant. The effect of MgATP on processes (B) and (C) can be interpreted in the context of the biochemical evidence, in which MgATP enters the cross-bridge cycle after the desorption of the product, and the binding of MgATP to rigorlike cross-bridges promotes a rapid dissociation of actomyosin (Lymn and Taylor, 1971. Biochemistry.10:4617-4624.). The effect is not predicted by a model for force generation in which head rotation dominates the fast component ("stage 2" of Huxley and Simmons, 1971. Nature (Lond.).233:533-538. and 1973. Cold Spring Harbor Symp. Quant. Biol.37:669-680.), and head dissociation dominates the slow component ("phase 4" of Huxley, 1974. J. Physiol. (Lond.).243:1-43; Julian et al., 1974. Biophys. J.14: 546-562.).