SummaryYoung animals absorb and retain more calcium (Ca) than their older counterparts. The mechanism(s) for this age-related difference and the kinetics of intestinal calcium transport during maturation are not known. We determined, therefore, the unidirectional uptake and the transmural flux of [45Ca] in everted duodenal and jejunal sacs of suckling, weanling, adolescent, and adult rats using [3H] dextran as a marker of adherent mucosal volume. These measurements were carried out over a wide range of Ca concentrations (0.5-30 mM). Results indicate an evolving pattern of intestinal calcium transport with different kinetic characteristics emerging as the animals matured. The active component of transport became more pronounced with increasing age. In adult rats Km and Jmax of [45Ca] duodenal and jejunal uptake were several-fold greater than corresponding values for suckling rats. Transport at higher calcium concentrations (10- 30 mM) was non-saturable, and the' permeability coefficient decreased with age. The transition to a more saturable process occurred around the time of weaning. These findings suggest that intestinal calcium transport is characterized by a maturation pattern that starts with a predominantly passive system during infancy and changes to a saturable active mechanism during maturation.