Balance studies were carried out on four Suffolk-cross lambs which were fed a diet containing only 1-2 mg zinc/kg dry matter; zinc deficiency was induced within three weeks. After a period during which the deficiency was relieved by a pica, the zinc deficient state was re-established. Each sheep was then treated with a soluble glass bolus containing zinc, cobalt and selenium. The plasma zinc concentration of the sheep rapidly increased and was maintained for between six and 10 weeks. The bolus was able to supply the daily requirement of the sheep for zinc, with no detrimental effect on their copper status.