This study was conducted to assess the effects of age and dietary levels of copper and zinc on the intracellular distribution of these metals in sheep, the domestic species most susceptible to copper toxicity. Hepatic copper concentration was lower in newborn lambs than in 30- and 60-day old lambs and its distribution in neonates differed significantly from that in the older animals and from that observed in newborn and adult rats. Sheep previously maintained on a low-copper diet for 50 days were then fed diets containing 2.2, 11.3 or 47.0 microgram Cu/g diet with and without zinc supplementation (543 or 46 microgram Zn/g diet, respectively) for 60 days. Ceruloplasmin activity, total plasma copper and hematocrit were lower in zinc-supplemented sheep. Hepatic copper concentration was not reduced by zinc supplementation but was increased with each increase in dietary copper; the distribution pattern was significantly altered as hepatic copper increased. Hepatic zinc concentration and distribution were not affected by diet. Sheep fed the highest level of copper had higher copper concentrations in the mucosa of the small intestine and in kidney cortex. The concentrations of zinc in the kidney and of copper and zinc in diaphragm muscle and bile were not affected by diet.