1. The basic mechanism underlying the structural vascular changes occurring in hypertension was studied in cultured aortic smooth muscle cells (SMC) obtained by an explant method from spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP) and compared with that in normotensive Wistar—Kyoto (WKY) rats. 2. The growth rate of SMC from SHR and SHRSP at the age of 2.5–11 months was greater than that of SMC from WKY rats even after repeated passages. 3. [ 3 H]Thymidine and [ 14 C]leucine incorporation, and ornithine decarboxylase (ODC) activity of SMC were increased in SHR and SHRSP in comparison with WKY rats. 4. The application of isoprenaline but not noradrenaline to the culture media increased ODC activity acutely in SMC from WKY rats and this increase was blocked by propranolol. 5. These results indicate that SMC from SHR and SHRSP are more prone to proliferate than those from WKY rats and that a β-adrenergic neurohumoral mechanism accelerates SMC growth independently of blood pressure.