Over twenty years ago, during the course of the early investigation into the chemistry and physiology of vitamin A, it was found that the potency of herbage was related to its carotene content and mainly as a result of the work of Moore, it was established that this carotene could be utilized by animals and converted into vitamin A. Since this recognition of the carotenes as provitamins, the problems of the mode and site of coverstion in the animal body have aroused the interset of many workers. Until recently it was conaidered that the liver was the main site of conversion. Apart from the somewhat equivocal results obtained from attempted in vitro conversions using liver preparations, this assumption was based mainly on the fact that the feeding of carotene to vitamin A-deficient animals resulted in the almost immediate appearances of the vitamin in the livers. At the same time little, if any, carotene appeared in the livers whereas the alimentary tracts contained relatively large amounts of carotene and little vitamin.