Ectopic bone formation was induced in 14 rats receiving 100 ppm fluoride in drinking water and in 14 control animals. Sections from ossicles removed after 14 and 20 days were sampled for stereological analysis. Bone volume density and bone volume were reduced in experimental animals on day 14 (p less than 0.05). This difference was no longer present after 20 days. On day 20, surface density and areas of formative surfaces were increased in the fluoride group (p less than 0.05). Osteoid seam thickness was higher in the fluoride group on both days (p less than 0.01). In conclusion, fluoride induced quantitative alterations in ectopic bone formation, and the presented model may prove a useful addendum to previous methods for investigation of fluoride effects on mineralization processes in vivo.