Copper toxicity occurred in North Ronaldsay sheep fed on a diet of terrestrial herbage relatively high in molybdenum. Five sheep were taken from the island of North Ronaldsay, Orkney to the veterinary investigation centre at Thurso and fed solely on the herbage of the laboratory paddocks, supplemented in winter by hay made from these paddocks. The level of copper, molybdenum, zinc and total sulphur in the terrestrial herbage was analysed, together with the seaweeds which form the major part of the diet on North Ronaldsay. Serum copper, vitamin B12 and serum aspartate aminotransferase levels were obtained. Four of the five sheep died on this diet of terrestrial herbage and had liver copper levels of 1379, 1723, 2279 and 2281 mg/kg. The susceptibility of the North Ronaldsay breed of Orkney sheep to copper poisoning when first introduced to a diet of purely terrestrial herbage is demonstrated by the high liver copper levels of the four dead sheep compared to the normal serum copper levels and unimpaired health of two other breeds of sheep and a north country Cheviot cross North Ronaldsay ram.