We measured pressure-volume curves in nine excised dog ventricles and stress-strain curves in two to five muscle specimens from each ventricle to verify a derived formula that relates muscle stiffness to the ventricular pressure-volume curve. The assumptions underlying this formula are: (1) the ventricle is a uniform spherical shell, (2) all muscle fibers carry average stress and deform as if they were at the midwall, (3) static equilibrium exists, (4) internal pressure induces the only load, and (5) the muscles exhibit an exponential stress-strain curve given by the equation sigma(epsilon) = alpha(ebeta epsilon - 1), where sigma = stress, epsilon = strain, and alpha and beta are constants. There was no significant difference between the stiffness constant, beta, inferred from the left ventricle pressure-volume curves (14+/-4.3[SD]) and that measured directly from the muscle stress-strain curves (16+/-2.8).