Diastolic dysfunction is accompanied by the stiffening of the left-ventricular myocardium and accounts for close to half of heart failures. We report the use of Lamb wave Dispersion Ultrasound Vibrometry (LDUV) technique to measure elasticity and viscosity of the left-ventricular free wall myocardium in a beating porcine heart. A curved linear array transducer was used to excite transient mechanical waves and track the tissue deformation. A Fourier space-based analysis of the tissue displacement yielded Lamb wave dispersion velocities over a frequency range 50-500 Hz. The dispersion data was fitted with the analytical Lamb wave model to estimate myocardial elasticity and viscosity. LDUV was used to perform ECG-gated open-chest and transthoracic in vivo measurements of group velocity, elasticity and viscosity throughout a single heart cycle. The results show changes in group velocity, elasticity and viscosity lag behind the R-wave by around 100 ms and increase from diastole to systole.