The Soret spectra of the dimeric hemoglobin from Scapharca and of horse myoglobin reconstituted with protoporphyrin IX and Zn-protoporphyrin IX have been measured over the range 290-80 K. With increase in temperature the Soret band broadens and shifts to a different extent depending on the protein and the presence of the metal. In the Zn-protoporphyrin IX derivatives the spectral changes are more marked in myoglobin than in the dimeric hemoglobin. In the protoporphyrin IX derivatives, in which the spectral changes are significantly reduced, the opposite is true. The data have been analyzed in terms of coupling of the protein low-frequency vibrational motions to the porphyrin electronic transition (V. Srajer et al., 1986, Phys. Rev. Lett. 57, 1267-1270; A. Di Pace et al., 1992, Biophys. J. 63, 475-484). The analysis indicates that the heme pocket of the dimeric hemoglobin is characterized by an unusual rigidity and that the metal plays a different role in the transmission of the protein motions to the heme moiety in the dimeric hemoglobin and in myoglobin. Static and dynamic fluorescence measurements carried out at room temperature are in line with these conclusions.