NADH-dependent nitrate reductase (E.C. 1.6.6.1) was ultrastructurally localized in norflurazon-treated and control soybean cotyledons [Glycine max (L.) Merr.] by a method based upon the increase in osmiophilia due to the formation of an azo dye. The reaction product was observed in small vesicles throughout the cytoplasm. An apparent transport of nitrite to the plastid, the site of nitrite reduction, may occur through fusion of the nitrite-containing vesicles with the chloroplast envelope. Plants grown in tungstate lacked nitrate reductase activity as measured by standard assay procedures, and showed no increase in osmiophilia, suggesting a degree of specificity of this cytochemical procedure.