An existing compartmental model of human calcium metabolism able to describe successfully the dynamics of stable and radioactive calcium administered both orally and intravenously, is extended to include the bone calcium and its protein matrix, collagen, the secretion and catabolism of the hormones calcitonin and parathyroid hormone, and the hormonal control of calcium excretion by the kidney. Data selected from a literature survey of the rates and calcium contents of the major intestinal secretions, and of sweat, are incorporated into the model. Allowance is made for the increased secretion of calcium into the intestine during meals. By means of a digital computer program (CAMET2) 15 simplified simulations are carried out, corresponding to a variety of real situations including calcium and ethylenediamine tetraacetic acid infusions, the removal of the parathyroid glands, fasting, a low calcium diet and the intestinal absorption of radioactive calcium. The results emphasize the importance of renal calcium excretion in the overall control of calcium homeostasis. The roles of the skeleton, on which much emphasis has been placed in the past, and of the soft tissue calcium in buffering perturbations to the plasma calcium are considered. Common methods of calculating the total calcium secreted into the gut and of the endogenous faecal calcium are simulated and sources of error exposed. A meal subsequent to an oral tracer dose is shown to depress tracer absorption. The responses to a range of dietary calcium intake are considered.