The left and right ventricles (LV, RV) of the heart are connected hemodynamically via the systemic and pulmonary circulations and joined mechanically by shared myocardial fibers and the interventricular septum, resulting in ventricular interdependence. Changes in volume, pressure, and contractility of one ventricle can affect the other by volume shifts between the two circulations and by displacement of the septum. Here, we present a spatially resolved, biventricular model of the heart that can be used to investigate the effects of LV-RV interactions. Changes in the shape of the LV, RV and septum are represented by a small number of time-dependent parameters. In combination with a closed-loop circulation, this results in a low-order, computationally efficient model system. The free wall of the LV is represented as part of a prolate spheroid, and the septum and the RV are modeled by non-axisymmetric deformations of the spheroidal shape. The model includes interventricular interactions occurring as a result of changes in preload and afterload, as well as those resulting from direct mechanical interactions mediated by the septum. Model parameters can be matched to fit realistic patient geometries from echocardiogram data. Ventricular function is analyzed in terms of preload recruitable stroke work (PRSW). With decreasing LV contractility, RV PRSW remains almost constant over a range of contractility, but decreases when contractility is severely impaired. In response to reduced RV contractility, LV PRSW decreases slightly. The model can be used to examine the circulatory and mechanical interactions of the ventricles in the context of cardiac disease.