Contraction and relaxation within the heart is controlled by the coordinated rise and fall of Ca2+ levels within the cytosol of cardiomyocytes. This requires Ca2+ to be moved between subcellular and extracellular compartments with each cardiac cycle, since Ca2+ is not destroyed or broken down by the excitation–contraction and relaxation cycles. Many cardiac pathologies alter Ca2+ homeostasis and can lead to impaired contraction, incomplete relaxation, and arrhythmias. Ca2+-sensitive fluorescent probes enable monitoring of Ca2+ cycling and can be used to detect pathological changes that can then be investigated using novel therapeutic strategies in isolated heart cells and tissues. This chapter will describe simultaneous measurements of intracellular Ca2+ using the ratiometric indicator fura-2/AM, and contractile force in isolated multicellular muscles (trabeculae) dissected from rat hearts, or from viable samples of human right atrial appendage.