Ultrastructural changes in the ventricular myocardial cells in rats fed a low protein diet were examined by electron microscopy. The most striking changes were observed in the I-band region of the sarcomeres, which occurred very occasionally in myofibrils. In the sarcomere affected the I-band region was often fractured and/or disintegrated on one side, leaving an extended space, while the opposing I-band region disappeared along with dislocation of the intact A-band toward the adjacent Z-line. This dislocation was presumably attributed to the elasticity of titins connecting between the end of thick filaments and the Z-line. Fractured I-band regions were often accompanied by the dilated sarcoplasmic reticulum in the close vicinity of them. In some myofibrils the streaming and/or disruption of the Z-line were occasionally observed where disarrangement of thick and thin myofilaments were usually present. The study suggests that the fracture of the I-band region, consisting of actin and titin filaments, and the streaming of the Z-line of myofibrils are due to a proteolytic action of calpain and/or cathepsin L, which are activated by leaked Ca2+ ion and/or by modification of internal circumstances of the cytoplasm induced by a low protein diet, thus resulting in a low cardiac output.