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Experiments are described demonstrating the passage of one-mc/sec continuous wave ultrasound through the heart and lungs at power levels of 100 mw/cm2 at the transducer terminals (a total of 1.25 watts). When the sound was directed through the region of the heart, the ultrasound was modulated by the moving intra-thoracic structures in synchronism with the heart beat. The records obtained were modified both by exercise and by amyl nitrite administered to the subject, but remained synchronous with the heart rate. Modulation of the ultrasound did not occur in two warm corpses. Sonic energy at the levels used to traverse the thorax did not affect a simultaneously recorded electrocardiogram. No deleterious effects have been observed on a subject whose heart was irradiated at 1 watt/cm2 and 3 watts/cm2 (totals of 12.5 and 37.5 watts, respectively) applied to transducer terminals at intervals over a period of one year (Appendix I). When continuous wave ultrasound was directed through a lung field clear of the heart, it was found that the attenuation varied 50 db between full inspiration and a lung emptying of 3400 cubic cm (0 db = full inspiration). In addition to direct transmission, sound is scattered throughout the thorax. The mechanics of the ultrasonic phenomena are described.