A technique has been developed for measuring two-dimensional strains in the left ventricle of the isolated arrested rat heart subjected to passive ventricular loading. The pressure-volume relationship was found in eight hearts during inflation of a left ventricular balloon. With the zero-pressure state as reference, in-plane strain components were determined using a triangle of ultrasonic dimension transducers (0.6-0.8 mm diameter) placed 3-6 mm apart in the midwall of the left ventricle. Mean circumferential (fiber) strain was larger than longitudinal (cross-fiber) strain (0.108 +/- 0.045, 0.055 +/- 0.045, respectively, at 11 mmHg), and shear strain (-0.048 +/- 0.029) was negative, consistent with left-handed torsion. The in-plane angle of greatest stretch was uniform with inflation (range = -26.5 degrees to -34.5 degrees). The equatorial region of the left ventricle was modeled with finite element analysis of a transversely isotropic thick-walled cylindrical shell subjected to internal loading and axial forces. The material parameters of an exponential strain energy function were optimized so that the least-squares difference between the predicted and the measured midwall strains was minimized. Material properties, stress and strain in the rat heart were compared to values predicted for the dog. In both species the tissue was stiffer in the fiber direction than in the cross-fiber direction. The ratio of fiber to cross-fiber stiffness was lower in the rat (2.50) than in the dog (5.24) at low loads and approximately equal at higher loads (1.63 and 1.39, respectively). The computational and experimental analyses showed that the larger shear strain and more nonuniform in-plane extension in the rat may be an indication of significantly different anisotropic material properties in these two species, and implies differences in the collagen ultrastructure.