We applied the pseudostrain energy function of Humphrey et al. [J. biomech. Engng 112, 333-346 (1990a, b)] to characterize the passive biaxial mechanical properties of the right ventricle free wall myocardium [Sacks and Chuong, J. biomech. Engng 115, 202-205 (1993)]. The myocardium was assumed to be incompressible, pseudoelastic, and transversely isotropic, with transmural variations in fiber orientation within test specimens accounted for by the strain energy function. Using nonlinear regression, material constants were determined for the right ventricle free wall myocardium from the sinus and conus regions. The pseudostrain energy function was found to model the biaxial mechanical data well (r2 > 0.99). Transmural variations in Cauchy stresses, as well as the magnitude of the in-plane shear stress, were found to be small. Although comparisons with the left ventricle midwall myocardium data [Humphrey et al., J. biomech. Engng 112, 340-346 (1990b)] show clear quantitative differences, there is an overall qualitative similarity in the mechanical behavior of ventricular myocardium.