The reports concerning the mechanism of vitamin D action are still limited in number. In this study one-day-old White Leghorn cockerels, weighing about 35 g were used. After 5 weeks feeding with vitamin D deficient diet, 25 μc of ^<32>P-phosphate was given either orally or intramusculary. The birds were killed and the left tibia and other tissues were collected and radioactivity was determined by usual Comar's method. The data showed the higher ^<32>P-labelled in the vitamin D_3-treated chicks and rather lower in the vitamin D-deficient ones.