A hybrid three-dimensional solid mathematical model of cardiac ventricular geometry was developed using magnetic resonance (MR) images of the in vivo canine heart. The modeling techniques were validated using MR images of an ex vivo heart and direct measurements of cardiac geometry and mass properties. A spin-echo MR sequence with in-plane resolution of 1.0 mm was used to image the canine heart in 11 short-axis planes at contiguous 5 mm intervals. Contours points on the epicardial, left ventricular (LV), and right ventricular (RV) boundaries were selected manually at each slice level. A boundary representation geometric model was constructed by fitting third-order nonuniform rational B-spline (NURBS) surfaces through each set of surface points. Validation was performed with a formalin-preserved ex vivo canine heart. Compared to the anatomic specimen (AS), volume errors of the ex vivo model were 0.3, 1.5, and 5.8% for the LV cavity, RV cavity, and total enclosed volumes, respectively. Comparison of cross-sectional areas of the AS and the model at 10 levels demonstrated mean model errors of 4.1, 2.5, and 2.9% for the LV, RV, and epicardial boundaries, respectively. The methodology is relatively noninvasive, provides acceptable geometric accuracy, and is clinically applicable.