Nitrate leaching from agricultural soils has frequently led to concentrations above the EU drinking-water standard. Most data originate from sandy soils. In this experiment the mechanism and magnitude of nitrate leaching from grassland on a heavy claysoil were investigated. In an experimental field, nitrogen input and nitrate output were measured. A bromide tracer was applied to study solute transport mechanisms. Nitrate transport occurred mainly via preferential flow through mesopores and macroporesresulting in small mobile-water volumes and high nitrate-leaching rates. Nitrate concentrations in the drain discharge were high. Models cannot adequately predict nitrate leaching in heavy clay soils.