Hemoglobin exhibits allosteric structural changes upon ligand binding due to the dynamic interactions between the ligand binding sites, the amino acids residues and some other solutes present under physiological conditions. In the present study, the dynamical and quaternary structural changes occurring in two unligated (deoxy-) T structures and two fully ligated (oxy-) R, R2 structures of adult human hemoglobin were investigated with molecular dynamics. It is shown that, in the submicrosecond time scale, there is no marked difference in the global dynamics of the amino acid residues in both the oxy- and the deoxy-forms of the individual structures. In addition, the R, R2 are relatively stable and do not present quaternary conformational changes within the time scale of our simulations, while the T structure is dynamically more flexible and exhibited the T -> R quaternary conformational transition, which is propagated by the relative rotation of the residues at the alpha(1)beta(2) and alpha(1)beta(2) interface.