Explants of dormant cells from carrot roots were activated by shaking in distilled water at 25°C. They were fixed in 3 per cent glutaraldehyde, post-fixed with 2 per cent osmium tetroxide, and embedded, in Spurr's resin. Changes in ultrastructure were seen in the nucleolus, nuclear envelope, and cytoplasm. The most striking were those in the nucleolus, which are interpreted as the movement of the nucleolar organizer, that part of the chromatin containing the genes for rRNA, to a deeper position in the nucleolus. This region of chromatin, called the L zone, due to its lightly staining characteristics also disperses with cell activation and becomes less clear in the nucleolus. The nucleolus also shows enlargement, especially of the granular zone, with the appearance of vacuoles.