Developmental changes in the synthesis of glycosaminoglycans and collagen in embryonic chick skins have been studied by labeling the materials with D-[1-3H]-glucosamine and L-[2,3-3H]-proline, respectively, in culture. In day 8 to day 20 embryos and 2-day-old chicks, hyaluronic acid was predominantly synthesized, with chondroitin sulfate, dermatan sulfate, and heparan sulfate as minor products. The syntheses of all these glycosaminoglycan components increased on days 14-16, when collagen synthesis also increased rapidly. The tissue deposition of hyaluronic acid and chondroitin sulfate, however, decreased continuously, while that of dermatan sulfate increased steadily. These results suggest that increased rates of turnover of hyaluronic acid and chondroitin sulfate are essential to initiate remodelling of the tissue. Rapid increases in the synthesis and tissue deposition of hyaluronic acid occurred from day 20 to the newly hatched stage. This may be a response of the embryo to the coming change of environment from in ovo to ex ovo by carrying a large quantity of bound water in the tissue space. Type I collagen accounted for 68-85% of the newly synthesized collagens throughout the stages examined. Type III collagen was detectable from earlier stages (0.2-1.0%), and started to increase on day 14, but then increased sharply from day 17 to the newly hatched stage (8.6%) in parallel with an increased rate of collagen deposition in the tissue. Syntheses of type I trimer collagen and type X and Y chains with properties similar to those of alphaB and alphaA were also confirmed at all the stages examined, though the levels of the latter two components were only a few percent of the total collagen.