An at least partly implantable system for injecting a substance into a patient's body. The system comprises an infusion device for injecting a substance into a part of the body of the patient, the infusion device being adapted for implantation inside the patient's body. The system further comprises at least one reservoir comprising at least one compartment which accommodates and preserves or is adapted to accommodate and preserve the substance to be injected, the reservoir being adapted for implantation inside the patient's body in fluid connection with the infusion device to supply to the infusion device the substance to be injected into the patient's body. The system further comprises an active cooling device adapted for implantation inside the patient's body and adapted to cool the reservoir and thereby keep the substance within said at least one compartment of the reservoir at a temperature below 37° C.