Nitrogen metabolism in some selected upland crops was investigated using a stable isotope, ¹⁵N, as a tracer. An optical emission spectrometer as well as a mass spectrometer employed for the investigations made great advance in the detailed analyses. Characteristics of nitrate and ammonium uptake by roots and foliar uptake of nitrogen dioxide (an air pollutant) were clearly identified. The kinetic studies using ¹⁵N suggested that all the inorganic nitrogen be mainly assimilated into glutamine (particularly into amide), followed by transference to other amino acids and proteins in both the roots and shoots. Nitrate was partly reduced in roots and shoots, but largely kept in storage nitrate pools before reduction. Significant transfer of nitrogen from mature leaves to growing leaves and tops as well as to roots was demonstrated directly by ¹⁵N and by a compartment analysis.