The role of the internal water molecules in vasopressin and oxytocin receptors has been investigated via molecular dynamics simulations in hydrated membrane model. Several water molecules have been identified within the binding pockets of receptors, where they interact with the conserved residues. In all unliganded receptors, the water molecules bound to the highly conserved D2.50 cluster have been observed. It has been proposed which water molecules may significantly contribute to the stability of overall receptor structure. In receptor-ligand complexes the water molecules are involved in the receptor – ligand interactions by forming water-mediated hydrogen bonds at their contact surface.