Experiments were undertaken to study in vivo the effect of hexachloronaphthalene in vitamin A metabolism in the rat. The status of vitamin A in the animal was determined by tissue analysis for the vitamin and by visual observation for symptoms of vitamin A deficiency. Four studies were made. The effects of dietary hexachloronaphthalene on preformed residual vitamin A, on assimilation of dietary carotene, on assimilation of a single oral dose of carotene, and on the absorption and storage of a single oral dose of vitamin A palmitate were investigated. In addition, the effect of ingested hexachloronaphthalene on calf plasma protein was studied. The following conclusions were derived from the results of the experiments. 1. The metabolism of dietary hexachloronaphthalene resulted in an increased requirement for vitamin A in peripheral tissue. This effect was apparently separate from the liver hypertrophy and fatty infiltration which are characteristic results of chlorinated naphthalene ingestion in rats. The mechanism of this increased demand for vitamin A was not discerned. Vitamin E supplementation was or no apparent value in preventing this increase in the requirement for vitamin A. 2. The addition of vitamin E to carotene-rich diets normally produces an increased yield of vitamin A. This stimulus of carotene conversion was not apparent when hexachloronaphthalene was included in the ration. 3. The ingestion of dietary hexachloronaphthalene depressed the ability of the rat to convert carotene to vitamin A. This was not true when the total dose of the toxic compound was included in the carotene solution. The addition of a bile salt to the carotene solution was not or value in preventing the depression of carotene conversion. Neither, however, did it stimulate conversion in the normal animals as was expected. 4. The ingestion of dietary hexachloronaphthalene had no apparent effect on the ability of the rat to absorb and store a single oral dose of vitamin A palmitate. 5. Paper electrophoresis studies and analysis of changes in TCA precipitable plasma protein in calves indicated that a depression of the level of the plasma proteins, especially the albumins, was a manifestation of the ingestion of hexachloronaphthalene by these animals. / Master of Science