Equilibrium conformational fluctuation of Zn-substituted myoglobin (ZnMb) has been studied in the nanosecond to millisecond time region and 180- 300 K temperature region by the time-resolved transient hole-burning spectroscopy. In this technique, the conformational fluctuation of the protein is observed as the temporal variation of the hole spectrum burned by irradiation of the laser pulse. ZnMb solution samples in various solvent conditions were prepared and investigated to elucidate the solvent effect on the conformational dynamics of Mb. The configuration coordinate model assuming the harmonic energy landscape has given a fairly good description of the time dependence of the hole spectra. The observed temporal behavior of both the hole shift and the hole broadening was well expressed by the same stretched exponential correlation function with a rather small and almost temperature-independent β of 0.26. It was found that the correlation time τ(c) of the conformational fluctuation of ZnMb determined by this analysis depends linearly on the solvent viscosity regardless of the solvent composition and temperature. This means the almost 0 activation energy for the fluctuation process and can not be understood by simply assuming the Arrhenius-type crossing of the barriers separating the conformational substates. It is shown that this linear viscosity dependence of τ(c), as well as the temperature-independent β, is qualitatively explained in the framework of the hierarchically constrained dynamics (HCD) model [Palmer, R. G. et al. (1984) Phys. Rev. Lett. 53, 958-961] with the postulate that the dynamics in the lowest level in the HCD model corresponds in the actual system to the configuration fluctuations of the solvent molecules surrounding the protein.