3,4,5-trimethoxy-cinnamaldehyde (TMCA) caused nasal tumors in rats. These results can be important and may have some bearing on the high incidence of nasal tumors among woodworkers. Though it is not known whether TMCA as such is present in wood lignins, it may be formed metabolically in the animal body by the action of O-methyl-transferase from its phenolic congeners.