Rat liver and rabbit liver and kidney were exposed to focused ultrasound of 312 w/sq cm (rat) and 900 w/sq cm (rabbit) by direct application of the ultrasound transducer to the surface of the organ exposed. A discrete, narrow, cylindrical zone of tissue necrosis to a depth of approximately 2 cm could be induced in the tissue exposed to the ultrasound. Lesions thus produced were well tolerated and tended to be resorbed over a period of several weeks without evidence of hemorrhage, infection, or adverse systemic effect. In rabbit liver, a linear zone of necorsis was induced by linear movement of the transducer. Development of the use of local high-intensity ultrasound may broaden the clinical scope of thermal destruction of cancer beyond that accomplished by electrocoagulation or cryosurgery.