Disclosed is a drug delivery system from which a macrocyclic lactone can be released with a desirable zero order release profile. The system is based on the use of thermoplastic polymers, and particularly of polyethylene vinyl acetate copolymers (EVA). The drug delivery system of the invention comprises a solid non-porous reservoir in which the macrocyclic lactone is contained, preferably in a concentration well above the saturation concentration, and a non-porous skin covering the reservoir not initially loaded with the drug. The system is preferably in the form of a rod, wherein the core and the skin are concentric, and the end surfaces of the rod are not skin-covered.
