Manganese-substituted horse heart cytochrome c was modified at histidine-33 with the ruthenium complexes trans-(NH3)(4)(L)Ru-His33, where L=NH3, pyridine, or isonicotinamide. Manganese(II)-to-ruthenium(III) intramolecular electron-transfer rates were measured by pulse radiolysis as functions of concentration and temperature to obtain intramolecular and intermolecular electron-transfer rate constants and activation parameters.