Known and possible interactions between Ca, P, Mg, parathyroid hormone (PTH), calcitonin (CT), vitamin D and its metabolites, and interactions of each of these with other factors plus complexities and possible variations between and within domestic animal species, such as age, sex, physiologic state (i.e., pregnancy, lactation and growth) and diet, make Ca, P and Mg metabolism extremely dynamic and complex. Many advances have been made in understanding these interactions and how each of these factors is controlled, secreted or metabolized within the body. Some interactions among these factors are discussed in detail, mostly with a view to understanding mechanisms by which homeostasis of Ca, P and Mg is maintained. Also, three of the major diseases of mineral metabolism of cattle (milk fever, grass tetany and wheat pasture poisoning) provide excellent models to study the complex soil, plant and animal interrelationships among Ca, P, Mg and the factors that influence and control the ultimate utilization of these minerals in animals. These diseases are used to illustrate some of the interactions among many of the factors that influence Ca, Mg and P metabolism in domestic animals.