Abstract. Abstract. The sulfatcd glycosaminoglycans synthesized in the forelimb plates of rats on days 12, 13, 14, and 15 of gcstation were characterized by their susceptibility to various glycosaminoglycan lyases. On days 12 and 13, heparan sulrate accounted for approximately 65% of the newly synthcsized sulfated glycosaminoglycans. Small amounts of dcrmatan sulfate and chondroitin sulfates were also observed. On day 14, the relativc amount of chondroitin 4-sulfate began to increase, thcre being a compensatory decrease in the amount of heparan sulfate. 35S-Sulfate-labeled material was extracted rrom day-13 forelimb plates with 4 M guanidine/HCl without proteolysis. Using ultracentrifugation on a sucrose density gradient, the extract was separated into two peaks: a light peak (L) mainly composed of heparm sulfate, and a faster-sedimenting peak (M) mainly composed of chondroitin sulfate. The cartilage-type proteoglycan (H) was first detectable on day 14 of gestation, indicating that chondrogenesis in rat forelimb plates starts on day 14 of gestation. In addition to these previously iden- tified glycosaminoglycans or proteoglycans, we isolated an unknown component in the glycosaminoglycan preparations obtained from limb plates during these dcvelopmental stages. This component was not found in glycosaminogly- can preparations obtained cither from the brain or tail of rat fetuses at the same stages.