This protocol describes a robust and reproducible method for the preparation of highly viable adult ventricular myocardial slices from small and large mammalian hearts. Adult ventricular myocardial slices are 100-400μm-thick slices of living myocardium that retain the native multicellularity, architecture and physiology of the heart. This protocol provides a list of the equipment and reagents required alongside a detailed description of methodology for heart explantation, tissue preparation, slicing with a vibratome and handling of myocardial slices. Supplementary videos are included to visually demonstrate these steps. A number of critical steps are addressed that must be followed in order to prepare highly viable myocardial slices. These include: identification of myocardial fiber direction and fiber alignment within tissue block, careful temperature control, use of an excitation-contraction uncoupler and optimal vibratome settings and correctly handling myocardial slices. Many aspects of cardiac structure and function can be studied using myocardial slices in vitro. Typical results obtained from rat (small mammal) & human heart failure (large mammal) samples are shown, demonstrating myocardial slice viability, maximum contractility, Ca2+ handling & structure. This protocol can be completed in ~4 hours.