The relationships between blood pressure and hypertrophy in cardiac muscle cells and their nuclei were histometrically and microphotometrically examined in the M strain of stroke-prone spontaneously hypertensive rats (M-SHRSP). In 60-day-old M-SHRSP, hypertrophy of the cardiac muscle nuclei preceded increases in cardiac muscle cell diameters (MCD) and lengths (MCL). Increases in MCD started in the middle and deep layers of the lateral wall of the left ventricle. At 90 days of age, hypertrophy of cardiac muscle cells and nuclei occurred equally in the middle and deep layers of the left ventricle. Right ventricular hypertrophy began to appear after 90 days of age. In the middle and deep layers of the left ventricle and in the septum in 150-day-old M-SHRSP the MCL were 1.4 times those in 30-day-old M-SHRSP and the MCD were 1.8 times those in 30-day-old M-SHRSP.