T LEAST 90 per cent of all human kidney A stones have calcium as a major crystal constituent. The investigation of stone pathogenesis most frequently begins with determining whether an abnormality in calcium metabolism is a contributing factor. This report will review current information concerning factors which influence the excretion of calcium. The emphasis will be on the physiologic mechanisms involved rather than on a discussion of specific disease states. With the development of rapid and accurate methods for the determination of calcium in body fluids, the past ten years have brought important advances in our understanding of the interactions between calcium and the kidneys. These are of biologic importance not only because of their central role in certain disease states but also because they provide an intriguing model of the general relationship between calcium and membrane metabolism and transport.