A mathematical model of left ventricular (LV) function in the dog has been developed. Based on a time-varying systolic elastance, the fitting procedure combined a local search according to a second-order method with a global heuristic search and provided beat-to-beat uncertainty intervals for each of the model parameters. Experiments were performed on anesthetized open-chest dogs in which the LV pressure and aortic flow were measured. The model formulated for the LV elastance appeared to be satisfactory for different contracting conditions including isovolumic or auxobaric beats. The study was focused on the maximal isovolumic elastance which, unlike the end-systolic elastance, has been found to be independent of both the preload and afterload and closely dependent on the contractile state of the LV.