We studied the effect of heparin on proteoglycan synthesis by bovine aortic smooth muscle cells in culture. Confluent, growth-arrested cells were incubated with [35S]sulfate, [3H]glucosamine or [3]serine in the presence of 0-600 micrograms/ml heparin. Metabolically labeled proteoglycans secreted into the culture medium and associated with the cell layer were analyzed. In cultures treated with heparin there was a dose-dependent increase in [35S]sulfate incorporation into secreted proteoglycans which reached a maximum (35% above controls) at 100 micrograms/ml heparin. At higher concentrations of heparin, the stimulatory activity declined and finally disappeared. Radioactivity in cell-associated proteoglycans increased significantly (16% above controls) only in cultures treated with 100 micrograms/ml heparin. Heparin also produced similar increases in the incorporation of [3H]glucosamine and [3H]serine into secreted and cell-associated proteoglycans. While chondroitin sulfate, dermatan sulfate and heparan sulfate were elevated in the media, only chondroitin sulfate and heparan sulfate were increased in the cell layer. Heparin did not alter the degradation of proteoglycans. Heparin, while inhibiting the proliferation of subconfluent smooth muscle cells, also stimulated to a greater extent the incorporation of [35S]sulfate into proteoglycans. Other glycosaminoglycans, such as heparan sulfate, dermatan sulfate, heparin hexasaccharide and Sulodexide caused a significant but lesser stimulation of proteoglycan synthesis, while chondroitin sulfates and hyaluronic acid had no effect. Gel filtration chromatography of proteoglycans and their constituent glycosaminoglycans from heparin-treated and untreated cultures showed no differences in their molecular size. The results indicate that heparin can stimulate proteoglycan synthesis by vascular smooth muscle cells irrespective of their state of proliferation. This might have implications in vessel wall repair and arterial wall lipid deposition.