Discs cut from Jerusalem artichoke (Helianthus tuberosus) tuber tissue were shaken in distilled water at 25°C (termed ageing) for periods of 0, 3, and 24 h when samples were prepared for electron microscopy. Tissue samples were fixed with 3 per cent buffered glutaraldehyde followed by osmium tetroxide. The ultrastructure of the nucleolus changes significantly with ageing. By 3 h a region which we identify as nucleolar-organizing chromosome is beginning to move from an external position on the nucleolus into the fibrillar region. By 24 h this chromosomal region has become dispersed as small areas within the fibrillar zone. At the same time the nucleolus develops a large granular zone. These changes are discussed with reference to the known increase in RNA synthesis during the ageing process.