Abstract It is widely reported that cultured vascular smooth muscle cells (CVSMCs) from spontaneously hypertensive rats (SHR) show enhanced proliferation compared with cells from Wistar-Kyoto rats (WKY). The present studies were designed to find out whether this exaggerated proliferation in SHR is determined genetically and, if so, to evaluate the mechanism on the cell cycle. (1) Incorporation of [ 3H]thymidine into DNA was enhanced in CVSMCs from 3- and 12-week-old SHR compared with WKY but not in CVSMCs from DOCA-salt hypertensive rats compared with the cells from sham-operated rats. (2) DNA synthesis in SHR cells was enhanced further by addition of insulin (which is considered to be a progression factor) but not by arginine-vasopressin (AVP; considered to be a competence factor) or by angiotensin II (All). On the other hand, insulin, AVP and All significantly augmented DNA synthesis in WKY cells. (3) Intracellular free calcium concentration was slightly, but significantly, higher in SHR cells. (4) An increase in the population of DNA-synthesizing S-phase cells and decrease in (G2 + M)-phase cells in SHR were observed by flowcytometry. These data suggest (1) that enhanced DNA synthesis in CVSMCs from SHR is determined genetically, (2) that enhanced DNA synthesis in CVSMCs from SHR is largely dependent on an increased proportion of S-phase cells and (3) that this increase in S-phase cells in CVSMCs from SHR could be due to enhanced competence gene expression in SHR cells. (4) The increased intracellular free calcium concentration is compatible with an activation of the inositoltrisphosphate pathway.