Fine structure of the distal femoral epiphysis of growing mouse was studied by electron microscopy. The first morphological evidence of developing secondary center of ossification in the distal femoral epiphysis was found at newborn mouse. Ossification center was in the form of multiple foci of calcification and its cells were represented by remnant of degenerated cells within large lacunae that were separated by mineralized cartilaginous septa. Endochondral ossification beneath the articular cartilage proceeded in a less orderly manner than metaphyseal endochondral ossification. Columns of hypertrophied chondrocytes were not distinctly parallel to intercellular mineralized septa in all direction. Hypertrophied chondrocytes in the inner zone of the epiphseal center of ossification showed disintegrated. Resorption of mineralized cartilaginous septa was undertaken by perivascular cells and multinucleated chondroclasts. Resorption of the calcified cartilage was restricted to the region of ruffled border of the chondroclast. Growth along the metaphyseal side of the epiphyseal center of ossification was different from that along the articular surface. As the secondary center expanded toward the metaphyseal side, many vascular buds penetrated unmineralized cartilaginous septa and invaded viable chondrocytes. Many hypertrophied chondrocytes bodering the metaphyseal side of bone center remained viable after they became embedded in mineralized cartilaginous septa. This result suggested that the hypertrophied.