The femoral heads of young rats have been used to monitor changes in proteoglycan structure during growth and endochondral ossification. Proteoglycans were extracted in good yield. The tissue content of proteoglycans increased until the time of calcification and then decreased. In contrast, the collagen content increased over the period studied. On Day 20, just preceding the onset of calcification, the proteoglycans had a lower glycosaminoglycan content, were somewhat smaller, and contained a larger proportion of molecules that were not capable of interacting with hyaluronic acid. On Day 25, during ongoing calcification, the proportion of proteoglycans that were not capable of interacting with hyaluronic acid was low, while it again was high on Day 40, just preceding ossification. The relative glycosaminoglycan content of the proteoglycans was somewhat lower on Day 20 and Day 40. The results indicate that both at the time of calcification and at the time of ossification the proteoglycan structure changes, perhaps indicating a functional role for the proteoglycans in the calcification process.