A reverberation method is used to determine the total acoustic energy absorbed by a mouse when exposed to ultrasound at clinical frequencies. The specimen is exposed in a cylindrical stainless steel tank supported by wires to minimize acoustic losses using a diffuse field created by frequency modulating the 1.03 MHz carrier signal with white noise having a bandwidth of 285 kHz. In the reverberation method, the rate of change of acoustic intensity, also referred to as the decay rate, is measured using a piezoelectric probe when the source is modulated. The energy absorbed by the specimen is given by the product of the total acoustic input power, the exposure time, and the fractional increase in decay rate caused by the specimen. The decay rate for samples of castor oil enclosed in spherical glass vessels was measured at two temperatures, 20° and 30°C. The absorption coefficient was calculated from the decay rate by assuming the energy density within the specimen was uniform and equal to the average energy density within the tank. The results obtained were within 15% of previously reported values for castor oil. Anesthetized ICR mice were exposed using the apparatus and typical results are presented.