In our studies with SHR, WKY and Wistar rats, we observed normotensive WKY rats with cardiac hypertrophy determined by a greater left ventricular mass to body weight ratio (LVM/BW) than that of normotensive Wistar rats. Thus, we compared, in SHR, WKY and Wistar rats, the following parameters: LVM/BW, cell capacitance as index of total surface area of the myocytes, length, width and cross sectional area (CSA) of cardiac myocytes, left ventricular collagen volume fraction (CVF) and myocardial stiffness. The LVM/BW of WKY (2.41±0.03 mg/g, n=41) was intermediate between SHR (2.82±0.04 mg/g, n=47) and Wistar (1.98±0.04 mg/g,n=28). A positive correlation between blood pressure and left ventricular mass was found in SHR whereas no such relationship was observed in WKY or Wistar rats. Cell capacitance and CSA were not significantly different in SHR and WKY rats, being these values significantly higher than those of Wistar rats. The cell length was smaller but the width was similar in WKY in comparison to SHR. Papillary muscles isolated from the left ventricle of WKY and SHR were stiffer than those from Wistar rats. Consistently, a greater level of myocardial fibrosis was detected in WKY and SHR in comparison to Wistar rats. These findings demonstrate blood pressure-independent cardiac hypertrophy in normotensive WKY rats.