A new, completely in vivo method of measuring the rate of intestinal phosphate absorption has been developed. As expected from previous in vitro and ex vivo measurements, intestinal phosphate absorption is potently and rapidly stimulated by 1,25-dihydroxyvitamin D3. The response is saturated with as little as 11.3 ng of 1,25-dihydroxyvitamin D3 per day, consistent with a genomic mechanism. The effect of 1,25-dihydroxyvitamin D3 disappears when the dosing solution of phosphate is at 2 M, suggesting that 1,25-dihydroxyvitamin D3 stimulates active transport of phosphate but not diffusion of phosphate. Finally, unlike findings resulting from in vitro or ex vivo experiments, no evidence in vivo was obtained that phosphate absorption requires sodium or is inhibited by potassium.