In studies of myocardial performance of the intact heart, continuous measurement of the left ventricular septal-lateral diameter provides useful information. We describe a technique for placing a piezoelectric diameter gauge on the left ventricular endocardium of the intraventricular septum. This technique is simple and relatively atraumatic. It provides a way to evaluate septal-lateral dimension in either acute or chronic experiments and produces a stable signal that represents the instantaneous septal-lateral dimension of the left ventricle.