Between the incisor and molars in each dental quadrant, mice have a toothless gap (diastema) that may contain vestigial tooth primordia. It is still not clear whether suppression of odontogenesis in the mouse lower diastema can be attributed to epithelium, mesenchyme, or both. Therefore, using recombination experiments with mouse tissues from E11.5 and E13.5 stages, we investigated whether the epithelium or mesenchyme is responsible for the suppression of odontogenesis. Five groups of recombinants were established and cultured under mouse kidney capsules. The results demonstrated that at E11.5, the lower diastemal epithelium and mesenchyme possessed odontogenic potential and competence, respectively; at E13.5, both the lower diastemal epithelium and mesenchyme had odontogenic competence, while the lower diastemal mesenchyme did not possess odontogenic potential. On the basis of comparison of the odontogenic capabilities between the lower diastemal and molar tooth primordia, we conclude that mesenchyme is responsible for tooth regression in the mouse lower diastema.