The contents of 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) in plasma and intestinal mucose were increased by dietary calcium and by dietary phosphorus restriction. The concentration of intestinal occupied receptors for 1,25(OH) 2 D 3 was higher in calcium-restricted birds. The affinity (association constant) of intestinal receptors for 1,25(OH) 2 D 3 was lower in phosphorus-restricted chicks, as compared to control or calcium-restricted chicks. The number of binding sites were not influenced by dietary calcium or phosphorus restriction.