This article reviews the proliferative, structural and synthetic behaviour of vascular smooth muscle cells under a variety of conditions. It shows how some experimental procedures produce dramatic increases in smooth muscle cell proliferation and, in many cases, subsequent cell migration to the intimal layer. Possible control mechanisms influencing changes in such activity are discussed. The morphology, histogenesis and differentiation of vascular smooth muscle is reviewed, with particular emphasis on the differentiation of such cells into contractile or synthetic phenotypes. The significance of the synthetic phenotype is discussed in relation to the synthesis of intra- and extracellular components.