A delivery system (10) is disclosed for delivering a fluid-sensitive beneficial agent (7) such as a somatotropin, or an analogue or derivative thereof, to an animal such as a bovine. The delivery system comprises a wall that surrounds an internal compartment, said wall comprising a first wall section that limits the passage of fluid into the system and a second wall section that permits the passage of fluid into the system. An exit passageway is provided for delivering the beneficial agent an interior compartment (18) to the animal. The exit passageway is sealed with a wax seal (30) which is expelled through the passageway (13) as the internal pressure within the device (10) reaches a predetermined "bursting" pressure. The passageway (13) is preferably sized to compensate for slight variations in the efflux of the beneficial agent and to maintaining a sufficient volumetric outflow rate of beneficial agent in order to minimize influx of biological fluids from the external environment back into the device once the seal (30) is expelled.