The relative rates of synthesis of chondroitin 4-sulfate, chondroitin 6-sulfate, and hyaluronic acid by intact 8-day chick embryo vertebral cartilage are found to be influenced in complex fashion by varying concentrations of D-glucosamine the culture medium. At 0.17 mM D-glucosamine the intracellular pool of UDP-N-acetylhexosamine is less than 5 moles per mg of tissue protein and the rate of chondroitin sulfate synthesis is approximately twice the rate of chondroitin 4-sulfate synthesis. When the concentration of glucosamine in the culture medium is increased to 2 mM, the UDP-N-acetylhesosamine level rapidly comes to a steady state level of 30 to 35 nmoles per mg of protein and the overall of mucopolysaccharide synthesis is approximately doubled over the rate observed at the lower D-glucosamine level.  At 4 and 25 mM D-glucosamine, the  pool size of UDP-N-acetylhexosamine remains at 30 to 35 nmoles per mg of protein but the rate of synthesis of chondroitin 4-sulfate and chondroitin 6-sulfate is lowered, as is the rate of incorporation L-[³H]leucine into protein. The rate of hyaluronic acid synthesis, on the other hand, increases progressively as the D-glucosamine concentration is raised to 25 mM.  At 0.17 mM D-glucosamine, hyaluronic acid synthesis represents approximately 5% of the total mucopolysaccharide synthesis; at 25 mM D-glucosarnine, hyaluronic acid synthesis is 40% of the total mucopolysaccharide synthesis.  The incorporation of D-[¹⁴C]glucosamine into chondroitin 4-sulfate and chondroitin 6-sulfate proceeds with a lag. The lag is more pronounced for chondroitin 4-sulfate, and at all D-glucosamine concentrations chondroitin 6-sulfate is formed in excess of chondroitin 4-sulfate in the early stages of the incubation.  However, at a time in the incubation which varies with the D-glucosamine concentrations, the rate of chondroitin 4-sulfate synthesis begins to exceed the rate of chondroitin 6-sulfate synthesis and after a 20-hour incubation, the total amount of chondroitin 4-sulfate that accumulates is in excess of the amount of chondroitin 6-sulfate. At all D-glucosamine concentrations, the sum of the rates of chondroitin 4-sulfate Chondroitin 6-sulfate, and chondroitin synthesis is linear; thus, the change in rates of synthesis of individual mucopolysaccharides is not accompanied by a diminution in the over-all rate of mucopolysaccharide synthesis.  D-Galactosamine is incorporated into chondroitin sulfate at approximately one-tenth the rate of D-glucosamine incorporation. Increasing concentrations of D-galactosamine in the culture medium have the same qualitative effects on mucopolysaccharide synthesis as corresponding levels of D-glucosamine.