In seven arbitrarily selected groups of 3-, 5-, 9-, 11-, 15-, and 21-week and one-year-old spontaneously hypertensive rats (SHR) (F_25-26), qualitative as well as quantitative light and electron microscope studies were conducted on the myocardium in the left ventricular papillary muscle and lateral wall. Age-and body-weight matched groups of normotensive Wistar-Kyoto rats were employed as controls. In the strain of SHR used in this study, blood pressure was significantly higher than in the controls in the 7th week of age, reached sustained levels (approximately 180 mmHg) in the 15th week of age and remained higher thereafter. The heart weight/body weight ratio gradually increased in SHR after the 15th week of age. In 3-week-old SHR, there was no significant difference in fine structure of the myocardium in comparison with control animals. The cardiocytes were small in size and rapidly growing. Myofibrillogenesis seemed to occur in association with juvenile forms of intercalated discs, subsarcolemmal dense mats, occasional Z band expansions and unorganized filamentous structures in the periphery of preexisting myofibrils. In 5- and 9-week-old SHR, the muscle fiber diameters in the papillary muscle were significantly larger than those in control animals of the same age. Subcellular structures of many cardiocytes were, however, apparently equal in both papillary muscle and lateral wall of the left ventricle in SHR and control animals. In 11-week-old SHR, the cardiocytes were moderately hypertrophied in both papillary muscle and lateral wall. The myofibrils were significantly increased in mass in association with altered intercalated discs, extended subsarcolemmal dense mats and unorganized filamentous structures in the periphery of preexisting myofibrils. Focal interstitial fibroses occurred in in the myocardium, and tunnel capillaries appeared in some hypertrophied cardiocytes. In 15-week-old SHR, the cardiocytes had markedly hypertrophied in accordance with the establishment of sustained hypertension. The intracellular volume ratios of myofibrils and T system had increased significantly whereas the mitochondrial volume had markedly decreased resulting in a significant increase in the myofibril/mitochondria volume had ratio. There were marked changes in the intercalated discs associated with numerous immature sarcomeres, and frequent abnormal Z band expansions. Interstitial fibroses had markedly increased in number and extent. In 21-week-old SHR, in addition to findings similar to those in 15-week-old SHR, heterogeneous changes of the contractile material, mitochondria and sarcoplasmic matrix were more prominent, suggesting heterogeneous malfunctions in later stages of cardiac hypertrophy. In one-year-old SHR, subcellular changes in many cardiocytes qualitatively resembled those in 21-week-old SHR. Interstitial fibroses and tunnel capillaries were also increased in extent and number. In the lateral wall, myofibrils were partially malaligned so that they appeared to terminate at the free sarcolemmal membrane in hypertrophied cardiocytes. In the papillary muscle, intracellular volume fractions of myofibrils, mitochondria and T system tended to be normalized suggesting quantitative readaptation of the organelles in a steady state of hypertrophy.