The paper describes a new preventive maintenance approach for manufacturing systems under environment constraints. The manufacturing system under consideration consists of a machine M-1 that produces a single product in a Just-in-Time context. To satisfy a constant demand d, the system called upon another machine M-2 (the subcontractor), comprising the so-called environment, which produces at a certain rate the same type of product as M-1. Both machines are subjected to random failures. Whereas machine M-2 is uncontrollable from the maintenance point of view, an age-limit policy is used for preventive maintenance of machine M-1. It is proved that the best age to perform preventive maintenance depends on the history of machine M-1 and the state of M-2. A numerical example is used to illustrate the proposed approach.