Studies on the structural relaxation of myoglobin following CO photolysis revealed that the structural change of heme itself caused by the cleavage of the Fe-CO bond is completed within the instrumental response time (similar to2 ps) of the time-resolved resonance Raman apparatus used. In contrast, changes in the intensity and frequency of the iron-histidine stretching [v(Fe-His)] mode were found to occur in the picosecond regime. The v(Fe-His) band is absent for the CO-bound form, and its appearance upon photodissociation was not instantaneous in contrast with the changes observed in the vibrational modes of heme, suggesting appreciable time evolution of the Fe displacement from the heme plane. Same behaviors were observed for the model compound of the heme part without protein matrix. Therefore the intensity change in v(Fe-His) is not associated with protein relaxation following the CO photodissociation. The band position of the v(Fe-His) mode changed with a time constant of about 100 ps, whereas that of the model compound without the protein matrix showed no shift. This indicates that tertiary structural changes of the protein occurred in a 100 ps range.