A delivery system is disclosed for delivering a fluid-sensitive beneficial agent 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. The wall may further comprise an end cap which may include means for adapting the end cap for ultrasonic welding to the first wall section and means for maintaining the beneficial agent in contact with exit means. The compartment comprises a beneficial agent and an expandable means. The delivery system comprises exit means for delivering the beneficial agent to the animal. The exit means may include means for compensating for slight variations in the efflux rate of the beneficial agent and means for maintaining for a sufficient velocity or efflux rate of beneficial agent outward from the device while minimizing diffusion of fluids from the external environment back into the device.