This study evaluates physiological fluid heating during continuous bilateral insonation at 530 mW/cm2for 8 h in a bench simulation. It also examines the physiologic, histopathologic, and neurologic effects of bilateral Doppler imaging of middle cerebral artery (MCA) blood flow velocity using ultrasonic beams with 530 mW/cm2intensity in a canine model immediately after and 2 weeks after insonation. In saline-filled containers, instrumented with opposing Doppler probes angled 10 degrees off axis, temperature was recorded at 15-min intervals for approximately 8 h at the intersection of the Doppler probe axes. Three conditions were tested: 1) an ambient control, 2) continuous bilateral insonation at 530 mW/cm2per channel with the thermistor in position, and 3) intermittent thermistor insertion. In one group of canines, physiopathologic responses during continuous bilateral insonation of the MCAs for 8 h at 2 MHz and 530 mW/cm2were studied. Brains were prepared for histologic examination immediately after insonation. Cerebral temperature; arterial, venous, pulmonary artery, and capillary wedge pressures; electrocardiogram; cardiac output; MCA velocity; and arterial blood gases were monitored. In a second group of canines, a neurologic evaluation was performed before and after insonation and again after 2 weeks. Brain tissue was evaluated histologically after the last neurologic examination. Light microscopic study was used for all histologic evaluations. In the bench experiments, a net temperature rise in the fluid of the simulation amounted to 0.0075°C/h in the overlap region after correction for ambient temperature effects and artifact thermistor heating. In canines, brain temperature (after correction for core body temperature changes and artifact heating of the thermistor) rose a mean of 0.2°C (p < 0.05) by the first hour, thereafter unchanging. No significant changes in the physiologic, neurologic, or histologic evaluations were observed in either of the experimental groups.