A methodology for simultaneously measuring two-phase relative permeability and capillary pressure of porous rock samples from steady-state flow experiments, reported by Virnovsky et al. in 1995, is considered. The method requires measuring pressure drops in individual fluid phases across a rock sample. This paper studies the role of gravitational forces, neglected in the original derivation of the method. Direct numerical simulations of steady-state vertical how experiments, performed over a typical range of rocks and how parameters, show that the error caused by neglecting gravitational forces in the original method, is acceptable even for extremely high (similar to 10(-10) m(2)) permeability samples. Thus, if multiphase how measurements are performed on vertical samples, the gravitational force can be neglected for estimating relative permeability and capillary pressure. An extension of this method to the case of compressible fluids is presented.