Conformational changes in proteins have been observed to exhibit a nonexponential time course. In myoglobin the conformational relaxation that follows photodissociation of the heme ligand is a very extended process that stretches from less than 1 picosecond to nearly 1 microsecond. We explain these kinetics with a model in which the initial protein conformational substates are connected to the final substates and to each other via transition states of a single energy.