1. The effects of intravenous trithiomolybdate administration on the fate of 64Cu and on stable Cu metabolism were examined in sheep held in metabolism cages and fed on a low-Cu diet supplying 2·95 mg Cu/animal per d.
2. In Expt 1 the injection of trithiomolybdate, 30 mg molybdenum/sheep per d for 5 d, greatly increased plasma Cu levels and more than doubled the faecal Cu excretion. Urinary Cu excretion was unaffected. The disappearance from plasma of 64Cu injected during the trithiomolybdate administration was slowed although faecal 64Cu excretion was increased more than threefold.
3. In Expt 2 trithiomolybdate, 10 and 30 mg Mo/sheep per d for 3 d, was administered 22 h after the injection of 64Cu. Radioactivity reappeared immediately in plasma and faecal 64Cu excretion was increased.
4. In both experiments the 64Cu and the increased stable Cu in plasma were associated with albumin.
5. The experiments explain the effectiveness of thiomolybdates as ‘decoppering’ agents and provide a model for some of the systemic effects seen in ruminants exposed to increased dietary Mo. The experiments support the view that the thiomolybdates, by causing the appearance of new ligands, alter the distribution of Cu in tissues and cause an overall depletion.