Ammonium tetrathiomolybdate (TTM) is the treatment of choice for chronic Cu poisoning in sheep and is recommended in Wilson's disease. However, the long-term effects have not been fully evaluated and some evidence questions the long-term safety of the drug. The aim of the present study was to investigate the systemic distribution and retention of Cu and Mo in TTM-treated sheep of different breeds and Cu status. Low-Cu Cambridge sheep were divided into a TTM trial group (3.4 mg/kg, subcutaneously, on three alternate days per month, for 5 months) and a control group, and were killed at the end of the course or 7 months later. High-Cu sheep consisting of a Cu-supplemented (150 mg/kg) Cambridge group and a North Ronaldsay group were administered TTM as before and compared with untreated controls. Brain, liver, kidney, heart, skeletal muscle, pituitary, adrenals, tests and ovaries were retained for metal analysis. Mo accumulated in all organs including brain and pituitary (P < 0.02) in all TTM trial groups and was retained after cessation of treatment, except in liver, kidney and skeletal muscle. Cu was increased (P < 0.02) and retained in the cerebellum and medulla oblongata in the TTM-treated high-Cu Cambridge groups. Brain Cu v. Mo concentrations showed a strongly positive correlation (r. 0.7) in the high-Cu Ronaldsay group 7 months after TTM treatment. It is concluded that TTM is not all excreted but (Mo) is widely distributed and retained in many organs including brain and pituitary. In addition TTM may redistribute some displaced excess liver Cu (Cu-TTM) to the brain. The consequences of these disturbances await clarification.