Gravity-driven drainage of water from a column of glass beads of uniform size is studied using nuclear magnetic resonance (NMR). The evolution of proton magnetization and its spin-spin relaxation time is measured as a function of drainage time at different locations within the column. On the basis of these measurements a model for calculating water relative permeability directly from relaxation time data at various saturations, originally proposed by Chen et al. (1994), is successfully tested for the first time.