The objective of this study was to describe induced heterotopic bone formation in the rat using stereologic methods and to optimize the sampling for toxicologic studies. Bone formation was induced by implants of demineralized bone powder in 24 male Wistar rats. The ossicles formed were removed after 14, 16, 17, 18, and 20 days. From each ossicle, six sections were sampled equidistantly, and seven microscopic fields were analyzed in each section. Ossicle volume and weight decreased from Day 16 to Day 20. Bone volume density increased during the experimental period because of the reduction in ossicle volume, whereas the total bone volume did not change. Bone thickness increased from Day 14 to Day 20. No changes were observed in the surface density and total area of formative surfaces. The major part of the total variation was due to true variation between animals. The described histomorphometric methods may prove useful in quantitative toxicologic studies, e.g., of fluoride effects on cartilage and bone formation.