The validity of hydrogeological studies is often dependant on the correct estimation of the flux (or, equivalently, the equivalent transmissivity) of the field. However, depending on the level of fine-scale heterogeneity of the porous media considered, flow simulators based on the finite differences algorithm can be shown to underestimate the equivalent transmissivity. This result is shown to depend on the value on the intermesh transmissivities and not the mesh transmissivities. When working with geostatistically simulated point support transmissivities, intermesh transmissivities can be directly calculated so that the finite differences result obtained is consistent with the spatial transmissivity distribution. ¿ American Geophysical Union 1996