Ultraviolet microscopic spectrophotometry was used to investigate lignification in the secondary cell walls of wood fibers and vessel elements from either wild type or transgenic poplars with depressed activity of cinnamyl alcohol dehydrogenase (CAD). A distinct shoulder at 330 nm was characteristic of the transgenic poplar. Measurements performed after 1) a mild alkali treatment or 2) a reduction with sodium borohydride indicated that this shoulder merely resulted from the occurrence of conjugated carbonyl groups in the lignin polymer rather than alkali-soluble cell wall bound phenolic aldehydes. UV absorbance ratios (A330 /A280) measured in the center of secondary walls in the differentiating xylem clearly showed that the structural changes observed in the lignin polymer of transgenic trees occurred very early during lignin deposition. This suggests that, in poplar trees with low CAD activity, cinnamaldehydes are incorporated into lignin even at the early stages of lignification.