The term "bioavailability" attempts to include in a single concept the effect of a sequence of metabolic events, i.e., digestibility, solubilization, absorption, organ uptake and release, enzymatic transformation, secretion and excretion. Each of these events is difficult to measure experimentally, and, with the possible exception of digestibility and solubilization, all are age-dependent and subject to nutritional and hormonal controls. In the case of calcium, the body's demand increases and then decreases with age; it also increases with pregnancy and lactation. Age, pregnancy and lactation each affect the regulatable component of the intestinal absorption of calcium. The passive component of calcium absorption is a function of the amount of calcium solubilized and of intestinal transit time. Inasmuch as digestibility and solubilization of calcium are very difficult to determine separately, even a reliable measure of calcium absorption includes a measure of uncertainty. If one wishes to include in the term "calcium bioavailability" rates of net deposition in bone, as well as rates of excretion from the body, quantitative information on calcium pool size, turnover and the effects thereon of age, sex, endocrine and nutritional status are needed. In the case of other nutrients, rates of enzymatic transformation and organ utilization need also to be taken into account. It will therefore require major research programs before the term "bioavailability" of a nutrient can become a quantitative concept useful for clinical, nutritional or managerial evaluation and counseling.