The sheep model has been widely accepted for evaluating the in vivo performance of artificial valves. Introduction of transthoracic echocardiography revolutionized the evaluation of human cardiac valves, but in many cases, resolution of transthoracic studies in sheep is inadequate. The authors used transesophageal echocardiography to follow the performance of chronically implanted mitral prostheses. The technique has proven feasible and safe, requiring sedation only and being done in the spontaneously breathing animal without endotracheal intubation in the left lateral position. The HP 77020A, equipped with a standard HP 5 mHz transesophageal transducer probe, is used. The authors have had to modify the probe to allow for automatic cooling. Standard insertion techniques are used and the probe is positioned in the stomach, close to the gastroesophageal junction. They were able to assess leaflet motion, vegetations, and calcification and use color flow doppler to evaluate stenosis and regurgitation. The technique is convenient and permits a complete assessment of valve function in 10 minutes. Reproducibility has been confirmed. Validation of quantitative data will require further study.