A basal diet containing 6.2 p.p.m. copper, 0.8 p.p.m. molybdenum, and 0.04 per cent. inorganic sulphate was fed unsupplenlented to one group of sheep. Four other groups were fed on the same diet supplemented with ammonium molybdate to give a molybdenum content of 5.1 p.p.m. or with sodium sulphate to give an inorganic sulphate content of 0.40 per cent., or with both. Sheep on the unsupplemented basal diet showed no marked disturbance of their liver and blood copper concentrations. Sheep maintained for 50 weeks on the diet containing both molybdenum and sulphate supplements showed a progressive fall in liver and blood copper levels, and developed dystrophic wool and hypochromatrichia. On a restricted feed intake of the same diet, sheep displayed a similar fall in liver copper and similar wool changes, but maintained a normal blood copper level for a longer period before developing hypocupraemia. Hypochromatrichia appeared to develop in this group before hypocupraemia was evident. With only the sulphate supplement added to the basal diet, liver copper concentrations fell, but not to a level associated with hypocupraemia. With only the molybdenum supplement added to the basal diet, copper retention by the liver was depressed to a small, but significant extent compared with the controls. The diet of the groups showing evidence of hypocuprosis approximately reproduced the mean copper, molybdenum, and sulphate status of the feed in an area in New South Wales enzootically affected with hypocuprosis. It is concluded that hypocuprosis in this area at least is explicable in terms of copper-molybdenum-sulphate interaction.