Earlier studies have shown that a single high dose of fluoride induces subameloblastic cysts separated by morphologically unaltered ameloblasts in the developing rat molar. In the present investigation, the ultrastructure of both the ameloblasts forming the cystic wall, and the cells within the cystic lumina were studied by means of transmission electron microscopy. It was shown that 24 h after fluoride injection the ameloblasts of the cystic wall showed various degrees of cytoplasmic and nuclear alterations. Some cells displayed signs of necrosis as indicated by condensation of the chromatin. The cytoplasmic changes varied from altered organelle morphology to fragmentation and almost complete shedding of the whole cytoplasm. In the ameloblasts of the cystic wall secretory products accumulated intracellularly, in distended rough endoplasmatic reticulum, in vesicles of the Golgi region and extracellularly between ameloblasts as well as between cells in the stratum intermedium, indicating an altered matrix secretion. Electron lucent material, cell and cell fragments were found in the cystic lumina, the two latter apparently originating from the ameloblastic layer. The degenerative changes seemed to follow the normal pattern of cell degeneration.