Many people are dependent on artificial pacemakers to have a normal cardiac function. Due to this it is important to study the effects of pacing on cardiac function, as well as, to determine the best site to pace with an artificial pacemaker that yields the best cardiac performance. Tau (τ), the time constant of left ventricular relaxation, has been studied as one measure of effective cardiac function, where high τ has been associated with myocardial ischemia and hence a low τ is desirable. The objective of the current study was to create a program in MATLAB™ that estimates τ from left ventricular pressure (LVP) data, verify this program using synthesized data and calculate τ for physiological data. LVP data was collected from five canines under four pacing modes: left ventricular (LV), bi-ventricular (BV), right atrial (RA) and right ventricular (RV) at rates of 90 or 100 and 160 bpm. Four models of τ were used: 1. A semi-logarithmic, zero asymptote model (τ L), 2. A semi-logarithmic model using data from the first 40ms of the isovolumic relation (τ 40), 3. Exponential model with non-zero asymptote of left ventricular pressure (τ E) and 4. A derivative model with non-zero asymptote of left ventricular pressure (τ D). The program successfully loaded all data files and computed τ for all dogs, all pacing sites and all heart rates.