Zinc sulphate was added to the drinking water of Angus cows and calves grazing an area considered marginal for copper nutrition of cattle. Mean daily intakes of approximately 12-15 mg Zn/kg live-weight further depressed already low plasma copper concentrations to levels considered to indicate copper deficiency. The plasma copper concentrations in untreated animals increased during the trial indicating adequate copper intakes during this period, but the high zinc intakes of treated animals prevented this seasonal rise. Injection of copper glycinate (0.24 g/animal) alleviated the depression of plasma copper associated with the high zinc intakes. The zinc supplements had little effect on plasma zinc concentrations.