Abstract: This invention concerns the delivery of anti-parasitic agents to ruminant animals. More particularly, the invention concerns delivery of such agents in a controlled manner to enable the agent to have maximum effect on the parasite for longer times than is possible with conventional formulations. The composition of the invention comprises a benzimidazole, macrocyclic lactone, organophosphate, salicylanilide/substituted phenol, tetramisole or pyrimidine anti-parasitic agent dispersed in a medium the solubility characteristics of which are such as to ensure that, following oral administration, controlled amounts of the anti-parasitic agent become available to the parasite, either directly or by absorption into the ruminant blood plasma, during passage of the composition through the rumen, the abomasum and the intestine.