Calcium absorption in the large intestine of the rat was investigated in vivo. After a single injection of 45CaCl2 into the cecum, 26.0 +/- 2.5% (mean +/- SE, n = 9) of the 45CaCl2 injected disappeared. This absorption was modulated by 1,25-dihydroxyvitamin D3, increased to 64.0 +/- 4.2% under a low-Ca diet, and increased under low-Pi diet. In contrast, when the difference of nonradioactive Ca in the cecal content and the feces was measured, only 4.1 +/- 4.6% (not significant) was absorbed. Secretion of intravenously injected 45Ca into the lumen was small and not altered by any of the conditions tested. When cecum contents were placed into duodenal tied loops, 14 +/- 6.2% were absorbed in situ when 45Ca was given orally, whereas when 45Ca was directly added to the content 35.6 +/- 4.6% were absorbed (P less than 0.02). These results indicate that the large intestine has an important vitamin D-dependent Ca absorptive system detectable if 45Ca is injected into the cecum. However, it is not effective in vivo because the Ca arriving in the large intestine appears to be no longer in an absorbable form.