Studies on various aspects of carotenoid metabolism have been carried out in the Biochemistry Department now for a number of years. A considerable fillip was given this work in 1954 with the return of Dr. W.A. McGillivray from study leave overseas spent at the National Institute for Research in Dairying at Shinfield, Working in collaboration with Dr. S.K. Kon and Dr. S.Y. Thompson, two senior British workers in the field; and again in 1955 with the reciprocal visit of the latter worker to this laboratory for a period of some nine months. The work reported in this thesis was carried out in the department between April 1955 and June 1957 during which time two separate investigations on different aspects of carotenoid metabolism were undertaken. The first of these was concerned with a study of the utilization of parenterelly administered carotenoids, Particularly in small animals, and the other with experiments on factors affecting the carotenoid and vitamin A contents of milk fat, in particular, on factors affecting the summer decline in the carotenoid and vitamin A contents of New Zealand milk fat. In order to facilitate the presentation of the results of these investigations, the work is reported in two separate sections.