AbstractThis paper presents a summary of experimental studies which utilized the White Leghorn cockerel,Gallus domesticus, as a suitable avian model for a detailed analysis of the mode of action of the seco‐steroid vitamin D. It is now apparent that there exists a complex endocrine system which coordinates the metabolism of the parent vitamin D into a family of over 30 metabolites; the principal metabolites are 1,25(OH)2D3and 24R,25(OH)2D3, which together orchestrate the spectrum of biological responses attributable to vitamin D. Key advances in elucidation of the scope of vitamin D endocrine system include the tissue distribution of both its steroid receptor and its gene‐induced product, a 28,000 dalton calcium binding protein, termed calbindin‐D28k. To date no less than 23 tissues have been found to have specific 1,25(OH)2D3receptors; of these at least 10 were identified in avian studies. Similarly, nine avian tissues have been found to express the vitamin D‐induced calcium binding protein, calbindin‐D28k. These observations collectively demonstrate both the broad scope of the vitamin D endocrine system and the appropriateness of using avians as valid models for vitamin D endocrine research which has applicability and validity for mammals, incl