1. In tissue culture the growth characteristics of aortic smooth muscle cells isolated from spontaneously hypertensive rats (SHR) were compared with those of normotensive Wistar-Kyoto (WKY) rats. 2. Aortic smooth muscle cells from SHR exhibit enhanced proliferation when grown in the presence of low (1%) and moderate (5%) concentrations of fetal calf serum. 3. Cell quiescence in cultures of smooth muscle from SHR becomes apparent at cell densities approximately 20% higher than in cultures from WKY rats. 4. These different growth characteristics of smooth muscle between the two strains of rats may contribute to the early pre-hypertensive development of vascular hypertrophy in the SHR.