In order to obtain fundamental information about the developmental mechanisms of myocardial fibrosis in chronic hypertension, the hearts of male spontaneously hypertensive rats of the stroke-prone strain (SHRSP) and Wistar rats of the Kyoto strain (WKY) were histologically and histometrically examined. Fibrosis was a prominent histological feature of the hearts in SHRSP. In consisted of focal, interstitial, and perivascular fibrosis. For histometrical analysis the percentage areas of interstitial and perivascular fibrosis were calculated by using a color image processor. The percentage area of myocardial fibrosis increased with advancing age in both SHRSP and WKY. However, it was significantly higher in SHRSP than in WKY at 18 and 30 weeks of age. In SHRSP perivascular fibrosis of small arteries had already appeared at 8 weeks of age, while perivascular fibrosis of arterioles and interstitial fibrosis developed later. It is supposed that perivascular as well as interstitial fibrosis is induced by the exudation of some growth factors due to an increased vascular permeability. On the other hand, the focal fibrosis observed in old SHRSP is suspected to occur as a result of injury in myocardium due to stenosis or occlusion of vessels.