Quantification of the viscoelastic properties of the myocardium using ultrasound may aid with evaluation of the health of the heart. Shearwave Dispersion Ultrasound Vibrometry (SDUV) is a method that uses dispersion, or variation with frequency, of shear wave velocities to characterize the underlying viscoelastic material properties of soft tissue. We studied eight pigs in an open-chest preparation, using a mechanical actuator to create harmonic, propagating mechanical waves in the myocardial wall. The motion was tracked using a high frame rate acquisition sequence, typically 2500 Hz, with a Sonix RP system. The velocities of wave propagation were measured over the range 50-350 Hz in 50 Hz increments. Data were acquired over several cardiac cycles. Dispersion curves were fit with a viscoelastic, asymmetrical Lamb wave model to obtain estimates of the shear elasticity, 1, and viscosity, 2. We found excellent agreement between the elastic term measured by SDUV and the shear elastic modulus measured by pressure-segment length analysis. We demonstrated that SDUV measurements and Lamb wave theory allow us to estimate the variation of viscoelastic moduli of the myocardial walls in vivo throughout the course of the cardiac cycle.