A detailed survey with magnetotelluric measurements covering the period range 0.04-500 s was carried out to define the geoelectrical structure of Goynuk geothermal area, northwest Anatolia. The study area is not far from the well-known geothermal field around northwest Anatolia that has several potentially valuable minerals and hot springs. The resulting two-dimensional (2-D) geoelectric structures, with true resistivity values, obtained from transverse electric (TE), transverse magnetic (TM) and joint TE-TM data inversion were interpreted by considering geological cross-sections, gravity anomaly and direct current (dc) Schlumberger vertical electrical sounding (VES) data. The 2-D models of geoelectric structures obtained from these inversions clearly display the existence of three electrically conductive (similar to4-6 Omega m) zones Z1, Z2 and sZ. The origin of remarkably good conductivity within the two large zones Z1 and Z2 located from the depth of about 5 km to depths of more than 15 km, and the existence of a smaller zone sZ at between 4-7 km are explained by the circulation of hydrothermal fluids with low resistivity values (0.28-0.52 Omega m) and by the effects of a strong hydrothermal alteration in the rocks. A low magnetic anomaly with low intensity of about less than or similar to50 gamma under the middle of the Goynuk area is ascribed to the hydrothermal demagnetisation of the rocks due to geothermal activity. The sharp decrease in resistivity to very low values probably suggests that both zones Z1 and Z2 are highly permeable (porosity about 30%) and therefore saturated with underground geothermal fluids. The upper zone sZ may act as a corridor for hydrothermal circulation between both of the zones Z1 and Z2, and it is a prime target for further geothermal exploration. (C) 2004 Elsevier B.V. All rights reserved.