When fed with [14C] phenylalanine in the light, xylem tissues isolated from poplar stems were able to incorporate part of the radioactivity into both the guaiacyl and the syringyl residues of lignins. In the dark, only syringyl units were integrated into the polymer whereas the guaiacyl residues remained unlabeled. When a membrane perturber (isopropanol) was added to the incubation mixture, the label was incorporated into the guaiacyl units either in the light or in the dark. Conversely, a membrane stabilizer (CaCl2) prevented the labeling of the guaiacyl units even when the tissues were illuminated. These results suggest that light acts through the modification of membrane permeabilities, altering specifically the synthesis and the transport or the polymerization of guaiacyl-type units during the process of lignification.