Proteoglycans are important structural elements of the extracellular matrix, and may contribute to the dynamic architecture of the lung and also influence pulmonary gas and solute exchange. The potential for proteoglycans and glycosaminoglycans to modulate the synthesis and deposition of elastin, another important extracellular matrix component of the lung, has not been established. Therefore the effects of glycosaminoglycans on the steady-state level of elastin mRNA, the incorporation of [3H]valine into tropoelastin, the distribution of soluble elastin in the medium and cell layer, and insoluble elastin deposition have been examined using cultured neonatal rat lung fibroblasts. Heparin decreases the soluble elastin content of the culture medium while increasing the soluble elastin content of the cell layer. This altered partitioning of soluble elastin is associated with an increase in steady-state elastin mRNA and an increase in the deposition of insoluble elastin in the extracellular matrix. Some of these effects may result from the binding of heparin to soluble elastin at physiological concentrations of NaCl. The galactosamine-containing glycosaminoglycans, chondroitin sulfate and dermatan sulfate, differ from heparin in that they increase the quantity of soluble elastin in the culture medium and decrease the deposition of insoluble elastin in the extracellular matrix. Proteoglycans, which are present in most elastic tissues, may participate in the regulation of elastin synthesis and deposition during periods of new elastin formation.