Proteins are dynamic systems. Recent evidence demonstrates that they exist in a large number of conformational substates and can continuously move from one substate to another; motion of a small ligand inside a protein may be possible only through these conformational fluctuations. To test this idea, we study with flash photolysis the binding of CO to protoheme and O2 and CO to myoglobin in many different solvents. The standard evaluation of such experiments yields information only about the protein-solvent system. A novel approach is presented which permits conclusions concerning the protein: Data from all solvents are considered together, and the rates for transitions of the ligand over various barriers are studied as a function of temperature for fixed solvent viscosities. Results show that over a wide range in viscosity the transition rates in heme-CO are inversely proportional to the solvent viscosity and can consequently be described by the Kramers equation. The rates of O2 and CO in myoglobin also depend on the solvent viscosity and are most sensitive to the solvent at the lowest viscosity. Viscosity influences protein reactions even in aqueous solutions. The data dan be interpreted by a dynamic model in which transitions into and inside myoglobin are governed by fluctuations between conformational substates corresponding to closed and open pathways. Ligand motion thus is mainly controlled by gates and not by static potential barriers. Some characteristic parameters for the substates are determined, and they agree approximately with similar parameters found in Mössbauer experiments. As expected, the barrier parameters evaluated in the novel approach deviate markedly from the ones obtained by the conventional procedure. Comparison with model calculations or basic theories will be meaningful only with the new evaluation, and the method may be essential for many or possibly all biochemical reactions.