To assist in the identification of inner heart structures during ultrasonic cardiac diagnosis, various liquids have been injected through catheters into heart chambers. Each injection produces a cloud of ultrasonic echoes within the blood-filled catheterized chamber. An experimental procedure involving injection of fluids into a test chamber under controlled hydrostatic pressure has shown that cavitation is the primary source of these echoes. Acoustic impedance difference between the injected fluid and the blood can contribute to echo production but is insignificant for commonly used fluids injected under normal conditions. Turbulence resulting from the injection produced no detectable echoes. [This work was supported in part by a grant from the National Institutes of Health.]