The induction of enzymes in plants is a well-known fact. Especially the induction of nitrate reductase (Reduced-NAD(P): nitrate oxidoreductase, E.C. by nitrate has been thoroughly studied (Beevers & Hageman 1969). In our research on the influence of N02 on plant metabolism we observed that nitrate and nitrite were formed in plant tissue after fumigation with N02. This raised the possibility of nitrate reductase induction by N02. To verify this hypothesis experiments were carried out with plants devoid of nitrate reductase activity by culturing them on a medium which contained N solely in the form of NH3.