Addition of actinomycin D (or cordycepin, an alternative inhibitor of RNA synthesis) to cartilage cultures resulted in a first-order decrease in the rate of incorporation of [35S]sulphate into proteoglycan (half-life = 7.5 +/- 1.1 h). Addition of 1.0 mM-benzyl beta-D-xyloside relieved the initial inhibition of glycosaminoglycan synthesis induced by actinomycin D; however, after a lag of about 10 h the rate of xyloside-initiated glycosaminoglycan synthesis also decreased with apparent first-order kinetics (half-life = 7.1 +/- 1.8 h), which paralleled the decrease in the rate of core-protein-initiated glycosaminoglycan synthesis. The hydrodynamic size of the proteoglycans formed in the presence of actinomycin D remained essentially constant (Kav. 0.21-0.23), whereas the constituent glycosaminoglycan chains were larger than those formed by control cultures, which suggested that the core protein was substituted with fewer but larger glycosaminoglycan chains. Proteoglycans formed in the presence of beta-D-xyloside were significantly smaller (Kav. approximately 0.33) than those synthesized by control cultures, and were further diminished in size after exposure of cultures to actinomycin D. Glycosaminoglycan chains synthesized by these same cultures on to both core-protein and xyloside acceptors were also smaller than those of control cultures. The decrease in synthesis observed after exposure to actinomycin D was not reflected by any significant decrease in the activities of several glycosyltransferases involved in chondroitin sulphate synthesis (galactosyltransferase-I, galactosyltransferase-II, N-acetylgalactosaminyltransferase and glucuronosyltransferase-II).