Copper deficiency in cattle is a world-wide problem. A deterministic digital computer simulation of copper metabolism was applied to cattle and especially to their response to dietary copper. This simulation was verified by comparison with published data on copper metabolism of cattle. Copper loss under conditions of dietary deficiency was governed largely by the rate of hepatic loss of copper. Copper stores were replenished gradually when the animal was returned to a normal diet. The increased copper consumption associated with increased total digestible nutrients (TDN) during lactation compensated for loss of copper in milk. Cattle have a very efficient mechanism for extrahepatic copper homeostasis by storage of excess copper in the liver, which can then be withdrawn in future deficiencies.