An at least partly implantable system for injecting a substance into a patient's body, in particular a penis erection stimulation system, comprises one or more long, flexibly bendable infusion needles, the tip ends of which are disposed within and implanted along with at least one first housing, in particular adjacent the patient's left and right corpora cavernosa. The respective other end or ends of the infusion needles are disposed and implanted along with at least one second housing remote from the first housing. A reservoir and a pump are also implanted inside the patient's body to supply the infusion needle with infusion liquid. A drive unit also adapted for implantation inside the patient's body is arranged for advancing and retracting the infusion needle such that its tip end penetrates the at least one first housing's outer wall in at least one penetration area, more specifically at least in two different penetration areas either simultaneously or in immediate time succession, thereby injecting the substance into the patient's body.