The dynamics of the histological changes which occur in the distal half of the tibial portion of the embryonic chick tibiotarsus from day 8 to day 18 of incubation are correlated with the capacities of histologically distinct zones to incorporate isotopic precursors into mucopolysaccharides and collagen. At the distal end of the tibia, which abuts the suture line formed by the fusion of the two tarsals with the tibia, there persists throughout embryonic development a narrow band of small, round or oval, rapidly dividing chondrocytes which synthesize chondroitin SO4 and collagen at low rates. Just proximal to this zone is a broader zone of flattened, disc-shaped chondrocytes which divide more slowly and are extremely active in chondroitin SO4 and collagen synthesis. Proximal to the zone of flattened chondrocytes is a zone of non-dividing, hypertrophied chondrocytes which are large and round and increase continually in size going from the distal to the proximal end of the zone. The biosynthetic activities of the cells in this zone fall sharply with their distance from the zone of flattened chondrocytes. Finally, there is a fourth zone, the marrow cavity, formed by a proximal to distal disintegration of the hypertrophied chondrocytes, starting at mid-diaphysis. The marrow cavity is surrounded by a shell of periosteal and intra-membranous bone which extends to the distal end of the zone of hypertrophied chondrocytes. Our data suggest that as the tibiotarsus grows in length the small, round rapidly dividing cells of the tibia differentiate first to flattened chondrocytes which synthesize matrix at a high rate and ultimately to low activity, hypertrophying chondrocytes. This sequence proceeds in a linear fashion.