Teeth develop via a reciprocal induction between theectomesenchyme originating from the neural crest and theectodermal epithelium. During complete formation of thetooth morphology and structure, many cells proliferate,differentiate, and can be replaced with other structures.Apoptosis is a type of genetically-controlled cell death and abiological process arising at the cellular level duringdevelopment. To determine if apoptosis is an effectivemechanism for eliminating cells during tooth development,this process was examined in the rat mandible including thedeveloping molar teeth using the transferase-mediateddUTP-biotin nick labeling (TUNEL) method. The toothgerm of the mandibular first molar in the postnatal ratshowed a variety of morphological appearances from thebell stage to the crown stage. Strong TUNEL-positivereactivity was observed in the ameloblasts and cells of thestellate reticulum. Odontoblasts near the prospective cusparea also showed a TUNEL positive reaction and severalcells in the dental papilla, which are the forming pulp, werealso stained intensively in this assay. Our results thus showthat apoptosis may take place not only in epithelial-deriveddental organs but also in the mesenchyme-derived dentalpapilla. Hence, apoptosis may be an essential biologicalprocess in tooth development.