Using an in vivo perfusion technique we compared transport of calcium in segments of the proximal (duodenum + jejunum) and distal (ileum) small intestine and in the cecum + colon of suckling (2-week old), weanling (3-week-old) and adolescent (6-week-old) rats. The concentration of calcium (Ca) in the isotonic sodium chloride solution perfused through the intestinal segments was either below (0.4 and 1.4 mmoles/1) or above (3.2 mmoles/1) serum ionized calcium. In the suckling rat, in all three intestinal segments, the relationship between transport rate (mumoles/hr per g wet weight) and luminal Ca concentration was linear. Net secretion occurred when luminal Ca concentration was below serum ionized Ca, and net absorption at 3.2 mmoles/l luminal Ca. In the weanling rats the relationship between luminal Ca concentration and transport rate was curvilinear for all segments. In the adolescent rats the relationship was curvilinear in the proximal segment and linear in the distal segment and in the cecum + colon. Rates of transport in segments of the suckling rats were several fold greater than corresponding rates in the adolescent rats. The change in the relationship between transport rate and luminal Ca suggested that the mode of transport of Ca changes at the time of weaning from mainly a passive mechanism in the suckling period to mainly a carrier mediated mechanism in the adolescent rats.