Chronic copper (Cu) poisoning is a well recognized problem in sheep and there is evidence that the incidence is increasing as more intensive methods of production are adopted. The interaction between Cu, molybdenum (Mo) and sulphur (S) in ruminants is well known and diets supplemented by Mo and S have been used to reduce the incidence of Cu toxicity in sheep. It has been postulated that Mo reacts with sulphide in the rumen to form thiomolybdate (TM) and that this subsequently combines with dietary and/or tissue Cu to form complexes in which Cu is unavailable to the animal. We have evaluated the response of Cu-dosed sheep to the intravenous administration of TM.