In order to determine the feasibility of using the laying hen as a source of molybdenum containing eggs, a dose response experiment was conducted which involved the feeding of graded levels of molybdenum to White Rock laying hens. The feeding of 2000 ppm of molybdenum resulted in decreases in egg production and hen weight. Only egg production was decreased at 1000 ppm of molybdenum. The 500 ppm level of supplementation appeared to be most applicable to further studies in that little effect on either hen weight or egg production was noted. It was also observed that at this level the egg concentration of molybdenum plateaued at 16.4 ppm after 14 days of feeding the experimental ration. An experiment designed to assess the toxicity of the molybdenum included in the egg by the laying hen to the developing chick embryo permitted the estimation of the L.D.₁₀₀ to be about 17 ppm of molybdenum. This value agrees quite closely to the L.D.₁₀₀ of yolk sac injected molybdenum. The embryonic death response observed in this experiment did not occur during any specific period of development. Succinoxidase assays conducted to study the death response indicated that there was no difference in succinoxidase. It was further noted that there was a decline in lethal response in embryos from hens fed the molybdenum containing ration for periods in excess of three months. This decline was mediated by a decreased concentration of molybdenum in the eggs from these hens. The mode of inclusion of molybdenum in the egg, by the laying hen was studied. It appeared that the majority of the molybdenum is included during the maturation of the ova in the ovary. The high concentration of molybdenum observed in the uterus suggests that the shell be considered in future studies. A chemical fractionation of the eggs from the hens fed the molybdenum containing diet indicated that very little of the molybdenum was present in the lipid fraction. Twenty-five percent of the total egg molybdenum was found to be associated with the protein fraction. A rat assay coupled with further chemical procedures indicated that the molybdenum to protein bonding was not of a covalent nature. The observation of n growth depression in chicks which had been injected with molybdenum during their embryonic development was not corroborated by a more extensive study. Although this study was confounded by the possible existence of a copper deficiency, some evidence was obtained for the involvement of molybdenum in the copper metabolism of the chicks. It was surprising that no increase in the blood and liver molybdenum concentrations was observed when the diets containing either 500 or 1000 ppm of molybdenum were compared. The blood and liver copper concentrations were little affected by molybdenum supplementation. / Ph. D.