The cytoplasmic components of chondrocytes in the various zones of the costochondral junction of male Sprague-Dawley rats were quantitated from electron micrographs using the point-counting method. A progressive increase in the content of endoplasmic reticulum, mitochondria, lysosomes, and Golgi complexes from the zone of small-size cartilage cells through the zone of hypertrophic cells was observed, while the content of lipid bodies, vacuoles, and multivesicular bodies decreased progressively through these zones. From these studies it was concluded that the hypertrophic cells are metabolically very active. The last chondrocyte at the base of each cell column, in the zone of provisional calcification, shows fragmentation of the cell membrane and of the nuclear envelope with loss of all cytoplasmic components except mitochondria and a few scattered remnants of endoplasmic reticulum, clear evidence that the ultimate fate of the epiphyseal chondrocyte is death.