The effects of low protein diet on cardiac metabolic and structural changes subsequent to an extremely high pressure load were investigated. Spontaneously hypertensive rats(SHR) were divided into four groups, and fed the following diets for four weeks: 1) Group A: regular diet (23% protein) and water, 2) Group B: regular diet and 1% saline, 3) Group C: low protein diet (10% protein) and water, and 4) Group D: low protein diet and 1% saline. Two weeks after the start of feeding, there was no significant difference in the left ventricular ultrastructures between the corresponding regular and low protein diet groups. Four weeks after, degenerative findings such as streaming of Z lines, dilatations of smooth endoplasmic reticulum, and disarrangements of myofilaments appeared Group D, while in Group B electron microscopic findings indicated hypertrophy. Incorporation of [14C] leucine into the myocardial myosin B in Group D was significantly low, with a resulting fall of the LVdP/dtmax per integrated isometric pressure (IIP) and a rise of the left ventricular end-diastolic pressure (LVEDP), as compared to that in Group C at four weeks after the start of feeding. These observations suggest that, in the heart with an extremely high pressure load, low protein diet hinders the development of hypertrophy to the load with resulting heart failure.