1. The uptake, transport and secretion of calcium into the gastrointestinal tract of germ-free (GF) and conventional (CV) chicks was determined by a variety of in vivo and in vitro techniques using 45Ca as a marker following a standard fast-feed regimen.

2. Following an oral dose of Ca in conscious chicks, the transit time of digesta in the GF birds was appreciably faster than in the CV controls.

3. Although the proportion of the Ca dose absorbed was similar in both groups of chicks, the accumulation of Ca in the intestinal wall was much greater in CV birds. Distribution of the labelled Ca in plasma and bone was similar in both groups.

4. Following intravenous Ca injection in conscious chicks the Ca secretion into the intestinal lumen was similar in both groups, but the amount appearing in the gut wall was appreciably greater in CV chicks.

5. There were regional differences in the ability of the gut to absorb Ca. There was a tendency for Ca uptake to be greater in the GF chicks but only in the proximal ileum did this approach significance.

6. When the Ca was confined to an intestinal loop the uptake/g tissue was similar in the proximal ileum of both groups but significantly greater in the distal ileum of the GF group. Uptake/10 mm gut was similar in both groups at both the proximal and the distal site.

7. It is suggested that the absence of a microbial load does not affect the ability of the enterocyte to absorb Ca but the subsequent removal of the absorbed load from the intestinal wall is appreciably slower in the CV group.