A laser light source and a digital autocorrelator were employed in the study of the molecular dynamics of acto-heavy meromyosin during the splitting of ATP. Low protein concentrations were used, so that molecular and not gel properties were evident. The addition of Mg2+ to acto-heavy meromyosin solutions in the presence of ATP caused a marked widening of the spectrum at high scattering angles. No such change was observed when chemically inactivated heavy meromyosin was used, when actin was cross-linked or when the proteins were in a high ionic strength solution. The data can be interpreted in terms of pronounced change in flexibility of acto-heavy meromyosin induced by active mechanochemical coupling.