The role of chondrocyte mitochondria in endochondral ossification has been the subject of intensive investigation and controversy. The purpose of this study was to quantitate the endogenous calcium content and the maximum capacity for calcium accumulation and release in isolated mammalian growth-plate chondrocytes and hyaline-cartilage chondrocytes. The results indicated that the mitochondria of the isolated growth-plate and hyaline-cartilage chondrocytes possess a greater endogenous calcium content, a greater capacity for calcium accumulation, and a larger labile Ca+2 pool than do the mitochondria of hepatocytes. Growth-plate and hyaline-cartilage mitochondria had an endogenous calcium content of 908 and 142 nanomoles of Ca+2 per milligram of mitochondrial protein. The growth-plate mitochondria had a maximum calcium capacity of 5249 nanomoles of Ca+2 per milligram of mitochondrial protein. In comparison, the mitochondria of hepatocytes had a much smaller endogenous-calcium content and a smaller maximum Ca+2 capacity: twenty-one and 3262 nanomoles of Ca+2 per milligram of mitochondrial protein, respectively. The mitochondrial labile-calcium pool in both growth-plate and hyaline-cartilage chondrocytes was twofold greater than that in the mitochondria of hepatocytes. Chondrocyte mitochondria released approximately 2400 nanomoles of Ca+2 per milligram of mitochondrial protein, whereas hepatocyte mitochondria released 1200 nanomoles of Ca+2 per milligram. These results suggest that the chondrocyte mitochondria are specialized for calcium transport and are important in the calcification of the extracellular matrix of the growth plate.