Ruminant animals are supplied with biologically active substances such as hormones (as growth supplements) antibiotics, antibloat agents, anthelminthics and mineral trace elements by administering pellets, of a density of at least 2.5 and weight of at least 1 gm. containing the substance, the pellets lodging in the rumeno-reticular sac and being such that the substance is released from them over an extended period of time.  The pellets may be made of a compressed high-density substance, or of such a substance contained in a sheath allowing diffusion of the substance, or of a low-density substance embodied in a high-density matrix or contained in a sheath or compressed round a core.  Examples of pellets are (a) cobalt oxide or copper oxide bound with clay and baked, (b) cuprous iodide bound with polyvinyl acetate, (c) iodine in a polyethylene sheath, (d) cobalt oxide and iron powder bound with clay, (e) an open-ended metal tube containing the substance, e.g. a mixture containing procaine penicillin, (f) a closed-ended metal tube containing the substance which is released by a wick and (g) an iron rod or steel ball covered with cement containing stilboestrol.  Other active substances specified include dieldrin (and other systemic insecticides and parasiticides) antibiotics such as penicillin, chlortetracycline, tetracycline, oxytetracycline, sulphonamides such as sulphadimidine and sulphadiazine and anti-bloat agents such as methyl-silicones and polyoxyethylene derivatives of ricinoleic acid.  Other binders include phenol-formaldehyde resins, borax, plaster of paris; granulating agents such as sugar, starch and vegetable gums and lubricants such as graphite or stearic acid may also be present. Examples of the manufacture of the pellets and experiments showing the beneficial results from their administration are given.ALSO:Ruminant animals are supplied with biologically active substances such as hormones (as growth supplements) antibiotics, antibloat agents, anthelminthics and mineral trace elements by administering pellets, of a density of at least 2.5 and weight of at least 1 gm. containing the substance, the pellets lodging in the rumeno-reticular sac and being such that the substance is released from them over an extended period of time.  The pellets may be made of a compresed high-density substance, or of such a substance contained in a sheath allowing diffusion of the substance, or of a low-density substance embodied in a high-density matrix or contained in a sheath or compressed round a core.  Examples of pellets are (a) cobalt oxide or copper oxide bound with clay and backed, (b) cuprous iodide bound with polyvinyl acetate, (c) iodine in a polyethylene sheath, (d) cobalt oxide and iron powder bound with clay, (e) an open-ended metal tube containing the substance, e.g. a mixture containing procaine penicillin, (f) a closed-ended metal tube containing the substance which is released by a wick, and (g) an iron rod or steel ball covered with cement containing stilboestrol.  Other active substances specified include dieldrin (and other systemic insecticides and parasiticides), antibiotics such as penicillin, chlortetracycline, tetracycline, oxytetracycline, sulphonamides such as sulphadimidine and sulphadiazine and anti-bloat agents such as methyl-silicones and polyoxyethylene derivatives of ricinoleic acid. Other binders include phenol-formaldehyde resins, borax, plaster of Paris; granulating agents such as sugar, starch and vegetable gums and lubricants such as graphite or stearic acid may also be present. Examples of the manufacture of the pellets and experiments showing the beneficial results from their administration are given.