The feasibility and safety of transvenous echocardiography was assessed in vivo. A 5 MHz esophageal echo catheter (O.D = 6.8 mm, Aloka Inc.) was inserted into the right atrium from the jugular vein or femoral vein in 10 anesthetized dogs. Real-time, two-dimensional echocardiographic images were obtained by moving the catheter back and forth, rotating the catheter during manipulating the transducer head with hand controls. When the catheter was positioned at the high-right atrium, the aortic arch, ascending aorta, and pulmonary artery was visible. At the mid-right atrium, the tricuspid valves, right ventricle, aortic valves, pulmonic valves, left atrium and mitral valves were clearly documented. Left main and left circumflex coronary arteries were also identified at this position, and coronary blood flow of the left main was easily detected in all dogs by means of the Doppler method. The left ventricle was most clearly observed in four-chamber and short-axis views when the catheter was positioned at the low-right atrium. From this position, the proximal to midportion of the left anterior descending coronary artery was also visualized as a tubular material. In three dogs, left ventricul ar wall motion changes were continuously monitored during a transient myocardial ischemia or dobutamine infusion. This in vivo study demonstrates that transvenous echocardiography using a low frequency transducer catheter can be performed in dogs. High-quality images were obtained from all cardiac chambers, valves, and great vessels. Real-time ventricular wall motion analysis is also possible with this technique. This method may be especially useful for measuring coronary blood flow and monitoring left ventricular wall motion during interventional techniques at the cardiac catheterization laboratory.