Monte-Carlo simulation of the systems containing a stack of 6 complementary base pairs and 180 water molecules has been performed. Characteristic of the hydration shell structure in major and minor grooves has been found for the stacks of repeating A : U and G : C base pairs as well as alternating (A : U, U : A) and (G : C, C : G) ones. Probabilities of the formation of bridges, formed by 1, 2 and 3 water molecules, between hydrophilic centres of the bases have been estimated. One water molecule forms an H-bonded bridge between two adjacent hydrophilic centres with high probability if N...N, N...O or O...O distance between these centres is close to 4.3 A. Hydration shell structure was found to depend significantly on the stack sequence and configuration, while global hydration characteristics (average energy, the number of water-water and water-base H-bonds) are only slightly dependent on the stack sequence and configuration. For the stacks in A conformation the number of water molecules forming more than one H-bonds with the bases is greater in comparison with the stacks in B-like conformation. This result is discussed in connection with the concept of hydration economy during B to A transition.