Hydrogen bonds are essential elements in protein structure and dynamics. We show that such bonds are strongly dominated by entropic contributions of the environment that can be at least as important as energetic considerations. These contributions are global in that they are not due to the internal degrees of freedom. The molecular dynamics (MD) is carried out as a computer experiment and agrees very well with the known energies of hydrogen bonds, both in water and in the isolated molecule. The entropy in water as a solvent is found to modulate the hydrogen bond breaking and reformation rate by two orders of magnitude, in a negative direction. (C) 2008 Elsevier B. V. All rights reserved.