The myocardium is represented as a thick-walled elastic cylinder contracting symmetrically. The concept of body force (force/unit volume) is used to model the force developed by the myocardium in the radial direction during systolic contraction. It is shown that the radial force per unit area developed by the myocardium on its inner surface can be included in the equation describing the pressure-volume relation in the left ventricle. Application of the equations derived to experimental data describing the pressure-volume line in the Suga-Sagawa model is given. The results obtained seem to indicate that the body force developed by the myocardium in a normal ejecting contraction reaches its peak toward the end of the contraction phase and that the peak value is related to the peak isovolumic pressure when a quasistatic approximation of the systolic contraction is considered. Implication of the developed model for future studies in cardiac mechanics is also discussed.