Tracerglass is a phosphate-based glass which dissolves in an aqueous environment at a slow and steady rate in proportion to the surface area of the glass (Telfer, Zervas & Knott, 1983). Tracerglass contains the trace elements copper, cobalt and selenium as part of the glass structure and therefore as the glass dissolves these trace elements are released into the aqueous environment. When placed in the reticulo-rumen of a ruminant by means of the normal balling gun method the Tracer-glass will dissolve, release the elements into the reticulo-rumen and thus make these elements available for utilisation by the rumen microbes and the animal itself. The Tracerglass-L form is a tapered bullet of average diameter 14 mm, length 40 mm and weight of 18 g which has been produced for use in lambs from 6 to 8 weeks pf. age. The Tracerglass-L dissolves at a rate that will maintain its effectiveness in lambs for up to 6 months while the adult sheep and cattle Tracerglasses will last for 12 months.