Early in primary culture, arterial smooth-muscle cells undergo a transition from a contractile to a synthetic phenotype. This process includes the loss of myofilaments and of contractility. At the same time, an extensive rough endoplasmic reticulum and a large Golgi complex are formed, and active synthesis of DNA, RNA and proteins commences. In the present study, chemical and immunocytochemical methods were used to investigate the production of extracellular-matrix proteins in relation to this change in phenotypic properties. The results showed that the phase of rapid cellular proliferation that follows the structural modulation of smooth-muscle cells is associated with high rates of collagen and elastin synthesis, as measured by the incorporation of 3H-proline into 3H-hydroxyproline and 3H-valylproline, respectively. SDS-polyacrylamide gel electrophoresis and fluorography indicated that type-I collagen is the main collagen species synthesized by these cells. Smaller amounts of type-V collagen and (although not definitively identified) type-III collagen were also detected. Indirect immunofluorescence and immunoelectron microscopy demonstrated that smooth-muscle cells surround themselves with an incomplete basement membrane, containing laminin and type-IV collagen, and thin fibrils of type-I collagen. Adjacent to these fibrils, aggregates of amorphous, elastin-like material were also found. Our observations confirm and extend earlier notions of a close similarity between the behaviour of arterial smooth-muscle cells during in vitro cultivation and during the early stages of the formation of atherosclerotic lesions.