A methods and apparatuses for ablation or hyperthermic treatment of body tissue are disclosed, wherein the application of radio-frequency ablative or hyperthermic energy is accompanied by the infusion of a conductive solution into the tissue, such that a virtual electrode is created. The virtual electrode results from the increased conductivity of the tissue in the area being treated due to the presence of conductive solution, such that the area of tissue being treated is enlarged as compared with non-fluid-assisted application of energy. In one embodiment, a catheter having a hollow helical electrode/needle on the distal end thereof is provided, such that the electrode/needle can be screwed into the tissue to be treated. A conductive fluid, such as saline, saturated saline, or Ringer's solution, is infused into the tissue to be treated via the hollow electrode and a lumen extending along the length of the catheter. In another embodiment of the invention, an implantable infusion port for fluid-assisted ablation and/or hyperthermia is provided with an elongate catheter having a hollow electrode/needle on the distal end thereof. The port is subcutaneously implanted with the electrode/needle situated within the tissue to be treated. A percutaneous needle facilitates the introduction of conductive fluid and/or chemotherapeutic agents into the tissue via a lumen in the catheter and the hollow electrode/needle. The percutaneous needle also makes electrical contact with a conductor in the catheter, such that as fluid is being introduced into the tissue, radio-frequency energy may be applied to the tissue being treated.