Mature seeds of Tropaeolum majus L. contain the cell wall polysaccharide xyloglucan (amyloid), protein and lipid as storage substances. The transitory occurrence of starch during the process of seed development could be substantiated.[U-(14)C]-labelled xylose, glucose and glucuronic acid were fed to ripening seeds and the incorporation of radioactivity into xyloglucan, starch and the sugar nucleotide fraction of the cotyledons was determined. The results indicate that exogenous supplied xylose is not incorporated directly into xyloglucan, but is transformed to glucose before incorporation into xyloglucan and starch. Radioactivity from glucuronic acid was predominantly found in the xylose moiety of xyloglucan. Incubation of seeds with [6-(14)C]-labelled glucose resulted in an incorporation of labelled hexoses into amyloid and starch, whereas xylose residues of amyloid remained unlabelled.