We examined the role of ras proteins in mitogenesis and proliferation of A10 smooth muscle cells. An assay was developed in which a dominant negative ras gene could be transiently expressed in vascular smooth muscle cells. As opposed to cells transfected with a control plasmid, those transfected with a dominant negative ras expression plasmid exhibited a decrease in thymidine uptake in response to serum, platelet-derived growth factor, epidermal growth factor, fibroblast growth factor and thrombin stimulation. In addition, expression of dominant negative ras blocked smooth muscle cell proliferation as assessed by the number of surviving colonies after transfection and neomycin selection. Our data is the first to show that ras is essential for smooth muscle cell proliferation and is a mediator in multiple smooth muscle cell mitogenic signalling pathways.