1. Wether sheep were used, some of which were prepared with a Thiry-Vella loop of jejunum. The long-term use of these loops was ensured by regular perfusion with fresh nutrient solution.
2. The net calcium absorption rale from a Thiry-Vella loop of jejunum increased with increasing intraluminal Ca concentration and was increased by the addition of 1 α-hydroxy-cholecalciferol (3μg/l) to the loop fluid.
3. When the diet of sheep was changed from one which was normal in Ca to a diet low in Ca there was an increase in the efficiency of net Ca absorption from the jejunal loop. This dietary change was accompanied by an increase in the circulating concentration of 1,25-dihydroxy-cholecalciferol (1.25(OH)2D3).
4. An intravenous infusion of the Ca chelating agent EDTA increased the efficiency of net Ca absorption from the jejunal loop. The use of 47Ca demonstrated that this reflected an increase in the true absorption rate of Ca.
5. Dietary phosphorus deficiency reduced the efficiency of intestinal Ca absorption and was associated with a reduction in the plasma concentration of 1,25(OH)2D3.