Abstract: A needle assembly (N) for RF ablation of target tissue in e.g. the liver (L) of a patient comprises a radial array of electrodes (1) in the form of loops which can be expanded by feeding the proximal ends of the Y electrodes along the bore of the needle assembly. An RF power supply (2) is connection between the array of electrodes and a skin pad (P) in contact with the skin (S) of a patient and, in accordance with the invention, is biased by a DC power supply 4 which generates electro-osmotic and/or electrolytic effects in the target tissue. In particular, chloride ions are discharged to generate chlorine which has a potent cytocidal effect and water can be attracted to the array of electrodes by electro-osmosis, thereby alleviating the problems of low electrical and thermal conductivity encountered with conventional apparatus.