The turnover of cells within the olfactory epithelium of the lamprey Lampetra fluviatilis was investigated using tritiated thymidine followed by autoradiography. It was found that cell division occurred in three distinct regions of the olfactory lamellae. Two of these regions — a distal lamellar region and a proximal lamellar region occurred outside the sensory area proper, but appeared to “contribute” cells to the sensory area as well as giving rise to secretory or ciliated cells outside the sensory area. A third region of division occurrred at the base of the sensory area. Division of specialised basal or “blastema” cells in this region gives rise to cells that are confined to the sensory region of the lamellae. These findings are discussed in the light of previous studies on cell replacement within the olfactory epithelium.