This thesis, after a review of the various syndromes arising in sheep and in cattle through a reduction in the copper levels of their blood and organs, describes in four sections experiments carried out on certain aspects of copper metabolism in these ruminants. #Section I: Variations associated with the season and in some cases with the age of the animal have been found to occur in the blood copper concentration, liver copper concentration, total liver copper and liver weight of 200 Cheviot sheep of all ages, in a Scottish hill-sheep area. #Section II: The copper balances of sheep have been studied with and without supplementaryfeeding of Molybdenum salts, to ascertain whether molybdenum has any effect on the copper balance. No conclusive results were obtained. #Section III: Experiments are described showing how dozing with B.A.L. (British-Anti-Lewisite, 2:3 dimercaptopropanol) increases the urinary excretion of copper. #Section IV: Considerable elevations in the blood copper level of sheep have been obtained as a consequence of two types of infection given to the animal.