The symmetry properties of the permeability matrix in the macroscopic transport law for two-phase immiscible flow in porous media are investigated. The porous medium is treated as a single, closed thermodynamic system being forced by piston reservoirs. This construction is used to relate the Darcy fluxes to the time derivatives of the piston motion, and to identify the fluxes and forces in the Onsager sense. When the surface-tension forces that develop on the fluid interface are linear in the interface displacement, Onsager's theory is directly applicable and the permeability matrix must be symmetric. This argument is extended to show that reciprocity still holds when surfactants modify interface properties.