We studied the mechanical properties of the sarcoplasmic reticulum of rabbit skeletal white muscle by measuring the ultrasonic velocity and the absorption per wavelength as functions of temperature and ultrasonic frequency. The bulk modulus of sarcoplasmic reticulum was determined from the ultrasonic velocity to be 3.0×10¹⁰ dyn/cm², and the bulk viscosity calculated from the absorption was 5~6 poise at 37°C. The bulk modulus slightly decreased as the temperature increased. The bulk viscosity showed large decrease below 20°C and small change above 20°C. Arrhenius plot of the bulk viscosity revealed the change in the activation energy: 8 kcal/mol below 20°C and less than 4 kcal/mol above 20°C. Considering the fact that the activation energies of Ca⁺²-uptake and ATP hydrolysis show changes at about 20°C, we conclude that this result suggests the close relationship between the activities of membrane integral proteins and the viscosity of the membrane. The frequency dependence of the limiting numbers of ultrasonic velocity and absorption at 25°C showed a dispersion and a peak, respectively, showing that there is a relaxation in the sarcoplasmic reticulum membranes at about 25°C.