There is widespread recognition that small increments in herbage molybdenum (Mo) concentrations can induce hypocuprosis and a recent study indicated that preventative measures might also be affected: copper oxide needles afforded less protection to ewes on high Mo diets than those on low Mo diets (Suttle, 1981 a). The effect of dietary Mo on the ability of other oral and parenteral copper (Cu) sources to alleviate hypocupraemia was therefore examined. Groups of Blackface ewes were depleted for each experiment by feeding a semi-purified diet containing 1 — 1.3mg Cu/kg DM and 4 g sulphur (S)/kg DM. Molybdenum was added as Na2Mo04, to provide an additional 4 mg Mo/kg DM to the 0.5 — 0.8 mg/kg DM present in the basal diet.