1. A new method of destroying thrombi in blood vessels by ultrasound and simultaneously removing them is reported. 2. Experiments were performed in twenty dogs with artificial thrombi in the iliac and femoral arteries and veins. The length of the thrombi ranged from 4 to 5 cm and the age from 12 h to 10 days. The unit used consisted of a generator, a frequency counter, an ultrasonic transducer with the hollow waveguide and a vacuum pump. For protection of the vessel wall the guide is surrounded by a plastic tube. The ultrasonic frequency is 26-5 kHz, the amplitude 25-30 micronm. The time needed to destroy the thrombus by ultrasound and suck out the thrombotic material ranged from 2-5 to 5 min. 3. To check side effects, experiments were performed in forty-four dogs. No significant changes were seen in the fibrinolytic system or microscopic structure of the vessel wall after ultrasound.