Utilization of ¹⁵N in the studies on N transformation in upland soils was reviewed with emphasis placed on a ¹⁵N-NH₄+ isotope dilution method. Although this method is not extensively used at present, it is informative and useful for quantitative investigations on N dynamics in soils, because it is the only means which can determine the gross rates of mineralization and immobilization at the same time. It is of primary importance to examine which kinetics can adequately explain each process of N transformation. Although both the zero-order and the first-order kinetics are applicable, their applicability depends on various factors: e.g. concentration of substrate and time span of incubation. Remineralization of tagged-¹⁵N is another critical problem. However, the errors caused by remineralization can be practically avoided by shortening the incubation period.