THE ABILITY to measure and assess cardiac function is useful in both clinical and research situations. Interpretation of such measurements provides both diagnostic and prognostic information, in addition to allowing evaluation of therapeutic success. In cardiovascular research, information regarding cardiac drug efficacy, as well as comparisons of cardiovascular function between populations can be generated. Such determinations are frequently expensive and invasive, thus reducing their utility. This is a particular problem in veterinary medicine, in which financial issues play a major role in determining which, if any, procedures are to be employed to evaluate cardiac function.Noninvasive monitoring techniques are attractive and include apexcardiography, Doppler and conventional echocardiography, radionuclide imaging techniques, and systolic time intervals (STI).' Currently available echocardiographically derived indicators of cardiac performance include left ventricular shortening fraction, stroke volume, ejection fraction, cardiac output, and STI. The subject of this discussion, STI, can be determined noninvasively, reflect global left ventricular function, and are sensitive to a variety of disease processes and pharmacologic interventions. Because they are variably sensitive to several different factors (heart rate and