1. Seven experiments involving 191 calves and 40 cows were under- taken over a 2-year period into the effect of copper treatment of hypocupraemic calves on their subsequent performance. 2. Significant increases in mean live-weight gain of copper-treated calves ranging from 19·9 to 34·3 kg/head relative to untreated control groups were obtained in three cases. 3. The frequency of copper injection required to maintain plasma copper levels above 0·60 mg/1 varied from 6 to 12 weeks. 4. The live-weight gain and plasma copper concentration responses are discussed in relation to the pasture herbage concentrations of copper, molybdenum and sulphate.