A 6-month experiment with nine dietary treatments was conducted to determine amounts of S plus Mo supplements required to maintain normal hepatic concentrations of Cu and Mo and to prevent chronic Cu toxicity in lambs fed palm kernel cake (PKC) diets. All diets consisted of PKC supplemented with minerals and vitamins, and with appropriate amounts per kg DM of S (level 0 or level 1 = 1 g) as sodium sulfate and/or Mo (level 0; level 1 = 4 mg; level 2 = 8 mg; level 3 = 16 mg; level 4 = 32 mg) as ammonium molybdate to form treatments S0Mo1, S0Mo2, S0Mo3, S0Mo4, S1Mo0, S1Mo1, S1Mo2, S1Mo3 and S1Mo4.There was no effect (P>0·05) of dietary treatments on the growth performance of the lambs. The dietary supplement of 1 g S plus 8 mg Mo per kg dietary DM (treatment S1Mo2) prevented accumulation of Cu in the liver without elevation of the concentration of Mo (P>0·05). The treatments S0Mo1, S0Mo2 and S0Mo3 increased (P < 0·05) hepatic Cu concentrations from 376 μg/g DM to between 1090 and 1294 μg/g DM. Also, the treatments S1Mo3 and S1Mo4 resulted in higher (P < 0·05) hepatic Mo concentrations compared with the treatment S1Mo0. It was concluded that the dietary supplement of 1 g S plus 8 mg Mo/kg PKC DM added to the PKC used is sufficient to maintain normal hepatic concentrations of Cu and Mo and to prevent chronic Cu toxicity in sheep fed diets containing any amount of PKC.