To study the reported decline in intestinal calcium absorption with age, calcium active transport, immunoreactive calcium protein (CaBP) content, and alkaline phosphatase activity were measured in the intestine of two strains of rats aged 3-wk--20 mo. Calcium active transport, as measured by everted gut sacs from Sprague-Dawley rats, was greatest at 3 wk, but it declined rapidly with no active transport demonstrable at 3 mo or thereafter. CaBP content closely paralleled the decline in active transport, but alkaline phosphatase activity increased as active transport decreased. Intestinal adaptation to dietary calcium was studied by feeding high- and low-calcium diets to Fischer 344 rats aged 1.5--12 mo. In 1.5-mo-old rats fed a low-calcium diet, there was an increase in calcium active transport, CaBP content, and alkaline phosphatase activity relative to animals fed a high-calcium diet. However, the magnitude of this intestinal adaptation decreased with age until there was only marginal adaptation by 12 mo. The observed changes in calcium active transport with age and diet may be explained by the parallel changes in the vitamin D-dependent CaBP content of the intestine.