A limiting factor governing the germination of seeds in soil is the availability of water for seed hydration. Soil water potential is one of the main factors controlling the amount of water moving to the seed surface from the surrounding soil. This potential has been related to the rate of moisture uptake of wheat seed germinated in soil of constant osmotic and different matric potentials. A diffusion model is used to describe the rate of increase in seed moisture. From the moisture uptake model, a wetting constant, which is related to the diffusion coefficient, is defined. An hyperbolic relationship was found between the wetting constant and soil matric potential 011er the range -5 to -66 kPa.