Small intestinal calcium transport adapts to dietary calcium restriction, but conservation of calcium by the ileum is incomplete. To examine the role of large intestine in calcium homeostasis, weight-matched growing rats were placed on semisynthetic diets containing 1.2 or 0.02% calcium. After 3 to 4 weeks, net calcium movements in cecum and colon (large bowel excluding cecum and distal rectum) were studied in vivo by perfusing a 0.4, 0.8, or 3.4 mM calcium solution. In animals taking 1.2% calcium, the cecum absorbed calcium actively at 0.8 mM; the colon showed net secretion except at 3.4 mM. In calcium-restricted animals, the cecum absorbed at all concentrations; the colon reduced secretion at 0.4 mM, converted to net absorption at 0.8 mM, and greatly increased net absorption at 3.4 mM. These observations suggest that the large intestine may have a role in calcium homeostasis, particularly when dietary calcium is restricted.