Human choriogonadotropin (hCG) belongs to a family of heterodimeric glycoprotein hormones involved in reproduction. Over 75 ns of molecular dynamics simulations of this heterodimer and the free ␣ subunit were performed and validated by experimental information to arrive at a qualitative dynamical description of these molecules. A number of 5-ns simulations at 400°K describe a sufficiently stable heterodimer structure, whereas the free ␣ subunit shows the experimentally observed partial unfolding. From the main collective fluctuations of the free ␣ subunit, it can be derived that residues ␣35-55 form a domain that is highly flexible with respect to the other domain, which contains all five disulfide bonds. The apparent loss of secondary structure in the region ␣33-58 may very well be induced by this. Dynamic domains can also be determined from the hCG heterodimer simulations. The most important collective mode of motion shows that the flexibility of the ␣ subunit is reduced by concerted rotation with both the long loop and the determinant loop of the  subunit. The motion of the free ␣ subunit does not differ significantly from the motion it has in the hCG heterodimer, but the amplitudes along the most important eigenvectors are larger.