Dissipation of the decay heat generated by high-level radioactive waste without producing unacceptable temperatures is one of the main problems related to geological disposal. An in situ heating experiment has been carried out in a clay quarry in the area of Monterotondo in order to discover the temperature field and the thermal effects caused by simulated high-level radioactive waste emplaced in an argillaceous rock. The experiment has been carried out by feeding an electric heater buried 6.4 m deep in a clay formation and by measuring temperature rises in boreholes drilled between 50 and 200 cm from the thermal source. The theoretical temperature rises in the clay, calculated by means of the Belgian MPGST code, have been compared with the experimental results. The temperature rises measured in the clay agree quite well with the theoretical values and show that the clay is a homogeneous medium. It was concluded that (a) the thermal conduction code seems sufficiently accurate to forecast the temperature rise caused in the clay by decay heat generation and (b) the thermal conductivity deduced by a comparison between experimental and theoretical temperature rises ranges between 0.015 and 0.017 W·cm−1°C−1. A laboratory-automated method, using needle and cylindrical probes, has also been designed to measure the thermal conductivity of clay samples. The probes are stainless steel cylinders, containing a heating resistance and a thermistor to measure the temperature rise. The method has been used to measure the thermal conductivity of clay samples coming from different Italian quarries. The thermal conductivity measured on the clay sedimentation plane is higher than that measured along the direction perpendicular to it. The clay thermal conductivity decreases as the water content rises.