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Vertical, two-phase flow through natural porous media is affected by interfacial coupling. Such coupling may be of two types: viscous and capillary. In this study, macroscopic defining equations for the viscous and capillary coupling parameters have been constructed. Moreover, these defining equations, together with modified forms of Kalaydjian's transport equations, have been used to construct equations that can be used to analyze the effect of interfacial coupling on vertical, two-phase flow in natural porous media. These equations reveal that viscous coupling is dependent explicitly on mobility ratio, and that the effect of viscous coupling on vertical flow is bounded. Moreover, these equations show that capillary coupling depends explicitly on porosity. Furthermore, on the basis of the analysis carried out, it is argued that the effect of viscous coupling on vertical flow is small, and that the effect of capillary coupling on horizontal flow is an order of magnitude larger than that of viscous coupling. The data from two sets of gravity-driven, steady-state, cocurrent and countercurrent flow experiments were used to test the interfacial coupling theory. It was found that the experimentally determined and predicted values of the capillary coupling parameters were consistent for both the wetting and the nonwetting phase, provided proper account was taken of the contribution of the net buoyant force to the driving force for each phase. Because of a lack of sufficient experimental evidence, further experimental testing of the theory must be undertaken before it can be accepted with confidence. Even so, the new equations should be, because they show explicitly the role of mobility ratio and porosity, useful tools for improving our understanding of the effect of interfacial coupling on vertical, two-phase flow through natural porous media. [ABSTRACT FROM AUTHOR]