The changes of hemodynamics were measured in spontaneously hypertensive rats (SHR) of increasing ages. Male SHR and Wistar rats of the Kyoto strain (WKY) at 4, 12, 24 and 48 weeks of age were used. The right jugular vein and the left femoral artery were cannulated and a thermistor was placed in the ascending aorta. After 24-hour rest, heart rate (HR), mean arterial pressure (MAP) and cardiac output (CO) were measured. The ratio of left ventricular weight (LVMI) of 4 week-old SHR had already increased significantly when compared to WKY. The HR in 4-week-old SHR was significantly higher than WKY. The increased HR in young SHR indicates the hypersensitivity of the sympathetic nervous system. Increased CO in 4 week-old SHR was due to high HR. The ratio of heart work to left ventricular mass (HW/LVM) of SHR at all age groups was not different from that of WKY, although the ratio of heart work to body weight (HWI) had a tendency to rise in SHR as compared to that in WKY. Our conclusion is that the development of LVM adapts to HW.