Selenate, a sulfation inhibitor, blocks the synthesis of heparan sulfate and chondroitin sulfate by cultured endothelial cells. In contrast, selenate does not affect the production of hyaluronic acid, a nonsulfated glycosaminoglycan. No differences in molecular weight, [3H]glucosamine/[35S]sulfuric acid ratios, or disaccharide composition were observed when the heparan sulfate synthesized by selenate‐treated cells was compared with that of control cells. The absence of undersulfated chains in preparations from cultures exposed to selenate supports the concept that, in the intact cell, the polymerization of heparan sulfate might be dependent on the sulfation of the saccharide units added to the growing glycosaminoglycan chain.—Dietrich, C. P.; Nader, H. B.; Buonassisi, V.; Colburn, P. Inhibition of synthesis of heparan sulfate by selenate: possible dependence on sulfation for chain polymerization.FASEB J.2: 56‐59; 1988.