In the methods used for determination of the hydraulic behaviour at large scale of fractured rock masses, based on the simulation of flow in a fracture network, the mean flux and mean pressure gradient in the network are not rigorously determined. A method is given for deriving these quantities, in a heterogeneous permeable block, from pressure and flux values on the boundary of the block. A block conductivity tensor is then defined based on the condition of linear variation of the pressure on the boundary of the block. It is shown that this conductivity tensor is symmetric and positive definite. An example of application to a model of fractured medium is given.