Comfrey, a medicinal herb with healing properties that are attributed to allantoin, was studied in this work. The accumulation and metabolism of allantoin (ALN) and its degradation product, allantoic acid (ALA), were examined. ALN was the main ureide in leaves and roots, with young leaves showing the highest content. ALA was the predominant ureide in the xylem sap, and together with ALN represented 27% of the nitrogen (N) transported in the xylem. Amino acids were the most abundant Ncompound in the xylem sap with a high proportion of glutamine. [ 14C]Xanthine feeding experiments showed that ALN and ALA were actively formed in leaves and roots by degradation of xanthine. Both xanthine and uric acid were rapidly degraded to form ALN and ALA. 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