It was examined whether the localization of sudanophilia at the site of calcification of articular cartilage was similar to those of epiphyseal cartilage and mandibular condylar cartilage in the rats given strontium carbonate or not, using Irving's method (1960). In control group, the sudanophilia was seen at the boundary between radial zone and primary spongiosa, that is, the zone of calcified cartilage. The short column was formed in the radial zone and the fibrous zone was not observed. In low Ca group, the width of articular cartilage slightly enlarged and the trabeculae of bone in primary spongiosa were fine, compared with those in control group respectively. The localization of sudanophilia was similar to that in control group. In Sr group, the width of articular cartilage, particularly, radial zone enlarged remarkably, compared with control and low Ca groups. The cell sizes of chondrocytes in radial zone were different from those in control and low Ca groups. The arrangement of column of chondrocytes was irregular, being different from that in control group. A great deal of osteoid-like tissue was seen at the area corresponding to primary spongiosa. Sudanophilia was not seen at the area corresponding to the zone of calcified cartilage. From the facts stated above, it was suggested that the localization of sudanophilia was similar among epiphyseal cartilage plate, mandibular condylar cartilage and articular cartilage, and the sudanophilia is closely involved in the mechanism of normal calcification of hard tissues.