A sustained release capsule ( 8 ) adapted to be inserted into the rumen of a ruminant animal through its oesophagus and retained within the rumen over a prolonged period to continuously deliver a biologically active composition ( 32 ) carried in the capsule ( 8 ), comprising an elongated tubular body ( 10 ) comprising a tube ( 16 ) and an end cap ( 12 ) secured to one end thereof for substantially enclosing the biologically active composition ( 32 ), the other end of the capsule ( 8 ) being a delivery end ( 18 ), the body ( 10 ) having an opening ( 20 ) at the delivery end ( 18 ) of the capsule ( 8 ) for delivery of the composition ( 32 ) to the rumen, and a plurality of retention arms ( 44 ) attached to or formed integrally with the cap ( 12 ), the arms ( 44 ) being adapted to extend outwardly from the cap ( 12 ) for retaining the capsule ( 8 ) in the rumen, the arms ( 44 ) being resilient to enable them to bend from their outwardly extending positions toward the body ( 10 ) so that they lie alongside the body ( 10 ) allowing the capsule ( 8 ) to be inserted through the animal's oesophagus, the resilient arms ( 44 ) normally extending from the body ( 10 ) at an angle between 75° and 90° with respect to the axis of the tubular body ( 10 ), and for each arm ( 44 ) said cap ( 12 ) has an external curved surface ( 26 ) disposed with respect to the arm ( 44 ) so that when the arm ( 44 ) is bent toward the body ( 10 ), the arm ( 44 ) contacts the curved surface ( 26 ) and the curved surface ( 26 ) controls the bending of the arm ( 44 ) so that the arm ( 44 ) does not bend abruptly, said arms ( 44 ) being adapted to return to their outwardly extending positions when the capsule ( 8 ) reaches the rumen, characterised in that said cap ( 12 ) is secured to said one end by means of at least two circumferential beads ( 30 ) having at least one sealing ring ( 31 ) located therebetween.