Precursors of lignin biosynthesis, ferulic acid and sinapic acid labeled by3H or14C were administered to the differentiating xylem of poplar shoots. Biochemical interconversions between syringyl and guaiacyl type intermediates involved in the biosynthetic pathway were suppressed by administration of precursors in the dark. The process of heterogeneous deposition of guaiacyl and syringyl lignin on the cell wall was visualized by autoradiography. Guaiacyl lignin was deposited in the early stage of the xylem differentiation on the vessel wall followed by deposition of syringyl lignin on the fiber cell wall.