A pellet for administration to ruminants to supply magnesium and other biologically active material, for example, anthelmintics or antibiotics comrpises a tube of magnesium alloy (1) enclosing a core or filling (3). The core (3) typically comprises mainly iron shot as weighting material, a biologically active material if desired and an electrically conductive material such as carbon or graphite. The graphite ensures that the core (3) as a whole is conductive and provides a cathode electrically coupled with the magnesium alloy tube (1) to form a galvanic cell. Liquor in the rumen serves as an electrolyte to cause galvanic corrosion of the magnesium tube (1) to release the active material contained in the core (3). A protective coating (5) of resin is provided on the tube's exterior surface except on at least one end (4) so that the corrosion is limited to the exposed surfaces only. An alternative embodiment (Fig. 4) comprises a control magnesium alloy rod or shaft (11) around which a cylinder (13) of the electrically conductive component is pressed. The cylinder contain iron shot and graphite in similar proportions to the core (3) in the first embodiment and is also provided with a protective resin coating (12) to restrict corrosion to one or both ends of the rod (11).