The conventional criteria of copper (Cu) status in ruminants, liver and blood or plasma Cu concentrations, show rapid reductions during depletion while the animal remains clinically normal. Criteria which change less rapidly in response to depletion might be more closely correlated with clinical and sub-clinical condition. Among such criteria are superoxide dismutase (SOD), which occurs as Cu- and Zn-containing enzyme in erythrocytes, and hair or fleece Cu. We have examined the temporal relationships between these different criteria of Cu status in two experiments, in which steers and lambs had been depleted of Cu, to assess their diagnostic value.