1. The ability of organic and inorganic sulphur to influence the copper and molybdenum metabolism of sheep was compared in a series of three 2 X 2 factorial design experiments. In each experiment, four groups of five to seven hypocupraemic ewes were repleted with a basal diet supplemented with 6 mg Cu/kg and containing S and Mo at one of two concentrations, 1 or 4 g S and 0-5 or 4-5 mg Mo/kg respectively. Sodium sulphate (Expt I), methionine (Expt 2) or cysteine (Expt 3) were used as the S sources. Cu and Mo concentrations in plasma were estimated in each experiment and in Expt 3 the concentrations of Cu in liver and Mo in urine were also estimated. 2. The effects of the three S sources on Cu and Mo metabolism were similar. Repletion of the plasma Cu pool was unaffected by Mo alone, reduced by S alone and totally inhibited by Mo+S. Plasma Mo was greatly increased by Mo supplements, slightly decreased by S supplements and unaffected by Mo and S supplements given together. 3. In Expt 3 the treatments were found to affect urinary Mo and plasma Mo in a similar manner; S prevented dietary Mo from increasing Mo excretion. The only group to show a significant repletion of the liver Cu pool was that given Mo alone. 4. Supplementation of the diet with organic S significantly reduced the within-treatment variation in plasma Cu and Mo. liver Cu and urinary Mo. 5. It is suggested that variations in dietary S and Mo within the normal range for herbage affect the Cu and Mo metabolism of the grazing animal, and that total S rather than inorganic S is the more useful measurement in the context of the Cu-Mo-S interrelationship.